# data[].attributes.body
Symmetry

Asymmetrical or radial

Parazoa

no true tissues

The fertilized egg undergo a cleavage: successive mitotic divisions into an organized multicellular form.

# data[].attributes.body
Radial symmetry

No ciliary combs

diploblastic

Gastrovascular cavity:

sac within a sac body plan

Cnidocysts

used to catch food

# data[].attributes.body
Acoelomates

tripoblastic

flatworms

tapeworms

flattened invertebrates

bilaterally symmetrical

lacks skeletal, specialized respiratory, and circulatory systems

no body cavity

have three embryonic layers

# data[].attributes.body
Pseudocoelomates- have an incomplete body cavity, improvise a complete one between a endoderm and mesoderm

have stomach, jaw, and anus

Lack a circulatory system

tough outer coat

many are parasites to plants and animals

proboscis in reduced body cavity

body cavity

bilateral symmetry

tissues

# data[].attributes.body
foot, mantle

ventral nerve cord or ganglion

no proboscis in body cavity

body cavity

bilateral symmetry

tissues

# data[].attributes.body
no jointed legs

segmentation

no foot or mantle

ventral nerve cord or ganglion

no proboscis in body cavity

body cavity

bilateral symmetry

tissues

# data[].attributes.body
jointed legs

segmentation

no foot or mantle

ventral nerve cord or ganglion

no proboscis in body cavity

body cavity

bilateral symmetry

tissues

# data[].attributes.body
no gill slits

no lophophore

nerve cord(s) absent or not ventral

no proboscis (nose) in body cavity

pentagonal symmetry (bilateral in larvae)

tissues

# data[].attributes.body
notochord

gill slits

no lophophore

nerve cord(s) absent or not ventral

no proboscis in body cavity

bilateral symmetry

tissues

# data[].attributes.body
Hexactinellida

Demospongia

Calcarea

Scleropongiae

# data[].attributes.body
Anthozoa

Hydrozoa

Scyphozoa

Cubomedusae

# data[].attributes.body
Planarians

Flukes (Trematoda)

Trubularians

Tapeworms

# data[].attributes.body
Monogononta

Bdelloidea

Seisonidea

# data[].attributes.body
Polyplacophora

Gastropods

Cephalopods

Bivalves

# data[].attributes.body
Oligochaeta

Polychaeta

Hirudinea

# data[].attributes.body
Crustaceans

Insects

Chelicerates

# data[].attributes.body
Sea stars

Brittle stars

Sea urchins

Sea cucumbers

Crinoids

# data[].attributes.body
Polyplacophora (chitons)

Chitons: marine, oval with shell divided into eight dorsal plates.

Grazers

Marine; shell with eight plates; foot used for locomotion; head reduced

Gastropods (snails, slugs)

Have shells

Grazers

marine, freshwater, or terrestrial; asymmetric body, usually with a coiled shell; shell reduced or absent in some; foot for locomotion; radula present

Cephalopods (squids, octopuses, chambered nautiluses)

Active predators

marine; head surrounded by grasping tentacles, usually with suckers; shell external, internal, or absent; mouth with or without radula; locomotion by jet propulsion using siphon made from mantle

Bivalvia (clams, mussels, scallops, oysters)

Marine and freshwater, flattened shell with two valves; head reduced; paired gills (allowed for gas exchange); most are filter-feeders (suspension feeders); mantle forms siphons

# data[].attributes.body
Urochordata (tail cords)

Cephalochordata (head cords)

Lancelet

# data[].attributes.body
Many consist of feathery or bushy colonies of tiny polyps. They attach to pilings, shells, seaweeds, and other surfaces. The polyps can be specialized for feeding, defense, or reproduction. Reproductive polyps produce small, transparent medusa. These are usually planktonic and release gametes into the water. The fertilized eggs develop into free-swimming planula larvae. Each planula settles on the bottom and develops into a polyp. The first polyp divides repeatedly and develops into a colony of many interconnected polyps.
Most marine, a few freshwater; both polyp and medusa stages in most species; polyp stage often colonial
(Portuguese man of war, hydras, Obelia, some corals)

# data[].attributes.body
Soft bodies invertebrates

Have a muscular foot that can be extended for movement

Two part, hinged shell

Complete, one-way digestive tract with a mouth & anus

Separate sexes

No eyes or distinct head

Have siphons to circulate water through their bodies

Filter feed on plankton

Mussels are large and have a flat, more oblong shell shape, while freshwater clams are smaller (3/4 inch) and typically more round

The animals of this phylum are soft-bodied, non-metameric, triploblastic coelomates and fundamentally bilaterally symmetrical invertebrates with a thin fleshy envelope the mantle, around the visceral (internal organs of the body) which may secrete calcareous shell. They are sluggish animals.

# data[].attributes.body
Planarians

lack organs for gas exchange

move by cilia located on their ventral surface

more complex nervous system

able to exchange behavior to stimulus

have two eye spots and lateral slits used for smell

asexual reproduction through regeneration

sexual reproduction but are hermaphrodites

Flukes (Trematoda)

parasites

life cycle integrates asexual and sexual reproduction

more tan one host is usually targeted

Trubularians

inhabit freshwater streams and marine type environments

Tapeworms

parasitic

live in vertebrates

absorb nutrients directly from the host

# data[].attributes.body
Monogononta

found mostly in freshwater but also in soil and marine environments. They include both free-swimming and sessile forms.

Bdelloidea

parthenogenetic reproduction

have the ability to survive dry, harsh environments by entering a state of desiccation-induced dormancy at any life stage

They are often referred to as "ancient asexuals" due to their unique asexual history that spans back to over 25 million years ago through fossil evidence. Bdelloid rotifers are microscopic organisms.

Seisonidea

found on the gills of Nebalia, a marine crustacean. Peculiar among rotifers, males and females are both present and equal in size.

# data[].attributes.attachment_link.title
Portuguese Man o' War: An Organism Made of Organisms?

# data[].attributes.body
Crustaceans (crabs, lobsters)

marine and freshwater, absent from land

Insects (dragon flies)

the largest group of hexapod invertebrates within the arthropod phylum

three pairs of legs

Chelicerates (sea spider)

primarily terrestrial

also horseshoe crabs and sea spiders

# data[].attributes.body
Urochordata

tunicates and seaquirts

marine bottom dwellers

sessile adults with a leathery tunic

larvae possess all 4 chordate trademarks

Cephalochordata

Lancelets (Amphioxus)

Marine

Fish-like shape, pointed at both ends

silver in color

small

closed circulatory system

# data[].attributes.body
have a sac body plan

flatworms have a single opening

tapeworms do not have a digestive tract

# data[].attributes.body
Sponges can grow into branching, tubular, round, or volcano-like masses that sometimes reach a huge size.

# data[].attributes.body
Oligochaeta (terrestrial and freshwater segmented worms)

reduced head; no parapodia, but setae present

Polychaeta (mostly marine segmented worms)

well-developed head; each segment usually has parapodia with setae; tube-dwelling and free-living

Hirudinea (leeches)

body usually flattened with reduced coelom and segmentation; setae absent; suckers at anterior and posterior ends; parasites, predators, and scavengers

# data[].attributes.body
All marine; medusa stage completely absent; sessile, many colonial.
(Sea anemones, most corals, sea fans)

# data[].attributes.body
Platyhelminthes reproduce sexually and disperse through eggs. Beyond that, there is little in the way of common life cycle features in this diverse group. Life cycles may be direct, or have 1, 2, or 3 intermediate hosts. Adults are usually parasites of vertebrate animals, while larval forms may be free-living or infect vertebrates or invertebrates. Some parasitic larval forms are capable of asexual proliferation. Life cycles may involve aquatic or terrestrial hosts, or both.

# data[].attributes.attachment_link.readable_text
Sea Anemones, Corals, Sea Pens

Daphne G. Fautin and Sandra L. Romano

Introduction

Anthozoans are exclusively marine, polypoid cnidarians. They include the familiar sea anemones, and other anemone-like groups with skeletons (such as the "stony" scleractinian corals) and without skeletons (such as tube anemones), as well as sea pens, sea fans, blue coral, and black coral. Anthozoans occur from the intertidal zone to the depths of the trenches (to 6000 m). In excess of 6000 species currently exist (Hyman 1940), comprising about two-thirds of extant cnidarian species (Dunn 1982); some anthozoans, such as the scleractinian corals, have a rich fossil history (Wells and Hill 1956).

An anthozoan polyp, like polyps in other cnidarian classes, consists of a tubular body terminating in a mouth surrounded by a ring of tentacles. The tentacles of an anthozoan polyp, unlike those of some other classes, are hollow, so the coelenteric space extends into them. The mesoglea between the ectoderm and the endoderm is relatively thick and cellular in many anthozoans compared to that of polyps in the other classes. The skeletal elements of some taxa are internal, lying in the thick mesogleal layer, whereas other anthozoans have skeletons lying entirely outside the living tissues (Fautin and Mariscal 1991).

Most anthozoan orders contain exclusively colonial species. An anthozoan colony consists of polyps connected by living tissue, the coenenchyme. The members of a colony are derived by vegetative (asexual) propagation from an original founder polyp that was the product of sexual reproduction, possibly with rare exceptions. At least some individuals of clonal species, like those of colonial species, propagate vegetatively by means such as fission, budding, or laceration, but, unlike in colonial species, the progeny do not remain attached to one another. Anthozoans of other species are incapable of asexual reproduction (e.g. Shick 1991).

Characteristics

Anthozoans are characterized by two anatomically related structures, the actinopharynx and the mesenteries, which are unique among cnidarian polyps (Figure 1). The actinopharynx, or stomodeum, is a tubular gullet extending from the mouth some distance into the coelenteron. The actinopharynx of most species contains at least one histologically specialized, flagellated longitudinal channel called a siphonoglyph that drives water into the coelenteron. In most sea anemones and corals, two siphonoglyphs are situated diametrically opposite one another in the actinopharynx. Siphonoglyphs and their associated structures impart a bilateral or biradial symmetry to the polyp (e.g. Shick 1991).

Longitudinal sheets of tissue termed mesenteries extend radially from the body wall; some ("complete" or "perfect" ones) reach all the way to the actinopharynx. The mesenteries that attach to the siphonoglyphs may be anatomically specialized. Longitudinal retractor muscles of the mesenteries enable the polyp to retract. Along the free edge of each mesentery may be filaments provided with cilia, gland cells, and cnidae. The cilia probably circulate fluid that otherwise might stagnate in the compartments defined by the mesenteries; the cnidae and gland cells function in digestion (Fautin and Mariscal 1991). The term "septum," which is sometimes used as a synonym for mesentery, properly refers to the calcified radial partition of the scleractinian skeleton that lies between a pair of mesenteries (Bayer et al. 1983).

It is inferred that the mesenteries increase surface area for respiration and especially absorption of food. They also provide support. Perhaps for these reasons, anthozoans of some species can grow much larger than polyps in any other class of Cnidaria -- to a diameter of 1.25 m for the "giant" sea anemone Stichodactyla mertensii (see Dunn 1981) (Title Figure 1). The gametogenic tissue of anthozoans lies between the longitudinal retractor muscle and the edge of a mesentery (but not all mesenteries of an individual may be gametogenic); gametes are derived from endodermal cells (eg Wedi and Dunn 1983). Polyps of some anthozoan species are gonochoric, those of some species are hermaphroditic, and those of still others change sex during their lifetime. Anthozoans of many species broadcast their gametes, but brooding of embryos within the coelenteron and attached to the external surface of the polyp is known (Fautin et al. 1989).

Variation within Anthozoa

Molecular data (France et al. 1996; Berntson et al. 1999; see Discussion of Phylogenetic Relationships below) support the division of the class into two groups, which are externally distinguished by the number and form of a polyp's tentacles. Subclass Alcyonaria (= Octocorallia) includes animals such as soft corals, sea fans, sea pens, and blue corals. Each polyp has eight tentacles that are pinnately branched (Title Figure 2); there are also eight complete mesenteries, so the space between each two mesenteries extends into one tentacle. The actinopharynx of each polyp has a single siphonoglyph. Alcyonarians of most species form colonies and skeletons. Mariscal (in Fautin and Mariscal 1991) proposed Alcyonaria be elevated to the rank of class. Extant members of subclass Zoantharia (= Hexacorallia) include sea anemones, "stony" corals, and black corals. Despite the name of the subclass, almost no members have six tentacles. Rather, zoantharians typically have (approximately) a multiple of six mesenteries and tentacles. Depending on the taxon, one or more than one tentacle may arise from each intermesenterial space.

Zoantharians fall into two clear groups: those in which mesenteries occur singly (Antipatharia [black corals] and Ceriantharia [tube anemones]), and those in which the mesenteries develop in pairs (all the others; see Figure 1). Black corals are the only exclusively colonial zoantharians.

Discussion of Phylogenetic Relationships

Class Anthozoa is traditionally considered to have two or three subclasses. Hyman (1940) divided the class into Alcyonaria and Zoantharia, based largely on polyp symmetry and tentacle form and number. Wells and Hill (1956) recognized a third subclass, Ceriantipatharia (listed also by Dunn 1982), based on similarity of the ceriantharian larval stage to the antipatharian polyp, on the very weak mesentery musculature in both groups, and on the insertion of new mesenterial couples only in the dorsal intermesenterial space of members of both orders. Evidence from morphology (Hand 1966), nematocysts (Hand 1966, Schmidt 1974), and DNA (France et al. 1996, Song and Won 1997, Berntson et al. 1999) does not support the monophyly of the Ceriantharia and Antipatharia. Indeed, Hand (1966) suggested division of Anthozoa into four subclasses (Antipatharia, Ceriantharia, Zoantharia, and Alcyonaria) could be justified. While the most complete molecular data available (Berntson et al. 1999) do not support the monophyly of the Ceriantharia and Antipatharia, placement of the Ceriantharia remains uncertain. The Ceriantharia may merit subclass status but more data are necessary to determine its phylogenetic position (Berntson et al. 1999).

We have chosen to follow Hyman (1940) in treating Antipatharia and Ceriantharia as orders of subclass Zoantharia. The zoantharian lineage has been interpreted as derived from the Alcyonaria, based on development (e.g. Hand 1966) and nematocysts (Schmidt 1974), but DNA sequences obtained thus far (Song et al. 1994, Bridge et al. 1995, Chen et al. 1995, France et al. 1996, Song and Won 1997, Berntson et al. 1999) support a sister relationship between Alcyonaria and Zoantharia.

Anthozoan Relationships Forum

The phylogenetic relationships of anthozoan subgroups are currently the subject of much debate. In the above Discussion of Phylogenetic Relationships, Daphne Fautin and Sandra Romano have given a summary of the most important issues. Many open questions remain, and there is some controversy about the conclusions that can be drawn from the data collected to date. In order to adequately represent alternative points of view in this discussion, we have established an Anthozoan Relationship Forum on a linked page.

Other Names for Anthozoa

anthozoans

Sea Anemones, Corals, Sea Pens

References

Bayer, F. M. 1956. Octocorallia. Pp. F166-F230 in: R. C. Moore (ed.), Treatise on Invertebrate Paleontology Part F: Coelenterata. Geological Society of America and University of Kansas Press, Lawrence.

Bayer, F. M., M. Grasshoff, and J. Verseveldt. 1983. Illustrated trilingual glossary of morphological and anatomical terms applied to Octocorallia. E. J. Brill / Dr. W.Backhuys, Leiden. 75 pp.

Berntson, E. A., S. C. France, and L. S. Mullineaux. 1999. Phylogenetic relationships within the Class Anthozoa (Phylum Cnidaria) based on nuclear 18S rDNA sequences. Molecular Phylogenetics and Evolution 13: 417-433.

Bridge, D., C. W. Cunningham, R. deSalle, and L. W. Buss. 1995. Class-level relationships in the phylum Cnidaria: Molecular and morphological evidence. Molecular Biology and Evolution 12: 679-689.

Chen, C. A., D. M. Odorico, M. ten Lohuis, J. E. N. Veron, and D. J. Miller. 1995. Systematic relationships within the Anthozoa (Cnidaria: Anthozoa) using the 5'-end of the 28S rDNA. Molecular Phylogeny and Evolution 4: 175-183.

Dunn, D. F. 1981. The clownfish sea anemones: Stichodactylidae (Coelenterata: Actiniaria) and other sea anemones symbiotic with pomacentrid fishes. Transactions of the American Philosophical Society 71(1): 1-115.

Dunn, D. F. 1982. Cnidaria. Pp. 669-706 in S. P. Parker (ed.), Synopsis and Classification of Living Organisms, volume 1. McGraw-Hill Book Company, New York and other cities.

Fautin, D. G. and R. N. Mariscal. 1991. Cnidaria: Anthozoa. Pp. 267-358 in F. W. Harrison and J. A. Westfall (eds.), Microscopic Anatomy of Invertebrates, volume 2: Placozoa, Porifera, Cnidaria, and Ctenophora. Wiley-Liss, Inc., New York and other cities.

Fautin, D. G., J. G. Spaulding, and F.-S. Chia. 1989. Cnidaria. Pp. 43-62 in K. G. Adiyodi and R. G. Adiyodi (eds.), Reproductive Biology of Invertebrates, volume 4, part A (Fertilization, Development, and Parental Care). Oxford and IBH Publ. Co., New Delhi.

France, S. C., P. E. Rosel, J. E. Agenbroad, L. S. Mullineaux, and T. D. Kocher. 1996. DNA sequence variation of mitochondrial large-subunit rRNA provides support for a two subclass organization of the Anthozoa (Cnidaria). Molecular Marine Biology and Biotechnology 5: 15-28.

Hand, C. 1966. On the evolution of the Actiniaria. Pages 135-146 in W. J. Rees (ed.), The Cnidaria and their Evolution. Academic Press, London and New York.

Hyman, L. 1940. The Invertebrates. McGraw-Hill, New York. 726 pp.

Hyman, L. H. 1956. Morphology of living coelenterates. Pp. F10-F20 in: R. C. Moore (ed.), Treatise on Invertebrate Paleontology Part F: Coelenterata. Geological Society of America and University of Kansas Press, Lawrence.

Oliver, W. A. J. and A. G. Coates. 1987. Phylum Cnidaria. Pp. 140-193 in R. S. Boardman, A. H. Cheetham, and A. J. Rowell (eds.), Fossil Invertebrates. Blackwell Scientific Publications, Palo Alto, California.

Schmidt, H. 1974. On evolution in the Anthozoa. Proceedings of the Second International Coral Reef Symposium 1: 533-560.

Schuchert, P. 1993. Phylogenetic analysis of the Cnidaria. Zeitschrift fuer zoologische Systematik und Evolutionsforschung 31: 161-173.

Shick, J. M. 1991. A Functional Biology of Sea Anemones. Chapman and Hall, London and other cities. 395 pp.

Song, J.-I., W. Kim, E. K. Kim, and J. Kim. 1994. Molecular phylogeny of anthozoans (phylum Cnidaria) based on the nucleotide sequences of 18S rRNA gene. Korean Journal of Zoology 37: 343-351.

Song, J.-I. and J. H. Won. 1997. Systematic relationship of the anthozoan orders based on the partial nuclear 18S rRNA sequences. Korean Journal of Biological Science 1: 43-52.

Wedi, S. E. and D. F. Dunn. 1983. Gametogenesis and reproductive periodicity of the subtidal sea anemone Urticina lofotensis (Coelenterata: Actiniaria) in California. Biological Bulletin 165: 458-472.

Wells, J. W. 1956. Scleractinia. Pp. F328-F440 in: R. C. Moore (ed.), Treatise on Invertebrate Paleontology Part F: Coelenterata. Geological Society of America and University of Kansas Press, Lawrence.

Wells, J. W. and D. Hill. 1956. Anthozoa - general features. Pp. F161-F165 in: R. C. Moore (ed.), Treatise on Invertebrate Paleontology Part F: Coelenterata. Geological Society of America and University of Kansas Press, Lawrence.

About This Page

Creation of this page was supported by NSF grant DEB95-21819 (PEET: Partnerships for Enhancing Expertise in Taxonomy) to DGF. Technical assistance was rendered by Chad Campbell and Adorian Ardelean. S. Romano acknowledges support from an NIH grant to R. H. Richmond in writing this page.

Daphne G. Fautin
University of Kansas, Lawrence, Kansas, USA

Sandra L. Romano
University of the Virgin Islands, St. Thomas, USVI

First online 04 October 2000

Content changed 03 October 2000

Citing this page:

Fautin, Daphne G. and Sandra L. Romano. 2000. Anthozoa. Sea Anemones, Corals, Sea Pens. Version 03 October 2000. http://tolweb.org/Anthozoa/17634/2000.10.03 in The Tree of Life Web Project, http://tolweb.org/

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Daphne G. Fautin and Sandra L. Romano Anthozoans are exclusively marine, polypoid cnidarians. They include the familiar sea anemones, and other anemone-like groups with skeletons (such as the "stony" scleractinian corals) and without skeletons (such as tube anemones), as well as sea pens, sea fans, blue coral, and black coral.

# data[].attributes.body
All marine; polyp stage reduced; free-swimming; medusas up to 2 m in diameter
(Jellies, sea wasp, sea nettle)

# data[].attributes.body
Cnidarians that have a small, mostly transparent, cube-shaped medusa with at least four tentacles.

# data[].attributes.body
Cnidarians are carnivorous and use tentacles to push food into their gastrovascular cavity.

Nematocysts are stinging cells that project needle shaped stingers into prey that inject poison killing or inhibiting them.

Types of Cnidarians
Polyp

sessile in location, these body types will sit and wait for prey

hydras and anemones

Medusas

flattened mouth down versions and passively drift with the current

sea jellies

# data[].attributes.body
The rotifer body is divided into four sections: head, neck, body and foot. The head supports a series of mobile tentacles called cilia, which propel water into the organism's mouth

The neck region contains a throat-like tube that funnels water into the rotifer's stomach and other organs, which are located in the body section. The foot protrudes from the organism's rear as an oar or spine to give the rotifer some control over its movement in liquid environments. Rotifers are bound by thin layers of clear skin, but many also have a harder shell called a lorica. Rotifers are invertebrates, so they have no spine or skeletal structure to maintain their shape.

# data[].attributes.body
Body has segments.

But the digestive system, nerve cords and circulatory system run the whole body.

Have a closed circulatory system

# data[].attributes.body
Epidermis produces a segmented, jointed and hardened chitinous exoskeleton with musculature between individual joints of appendages

Arthopod vision

simple eyes

light sensitive cells

compound eyes

thousands of closely packed units called ommatidia

Digestive system

three main regions: foregut, midgut and hidgut

respiration and circulatory system

Extensive surface areas are needed to supply the large oxygen demands of most arthropods

Gills can occur both inside and outside of the of the carapace

Circulatory System

Open circulatory system as in molluscs, hearts vary in structure but do exist

Blood is pumped into empty spaces bathing the tissues in blood

# data[].attributes.body
Notochord

Semi-rigid rod-like structure

Present at some time during development.

Made of cartilage

Functions:

Stiffening

Axis for muscle attachment

Hollow Dorsal Nerve Chord

Anterior end is brain

Is spinal chord in vertebrates

Pharyngeal Gill Slits or Pouches

Location: where pharynx will be

Present at some time during

life cycle

Post – Anal Tail

Function: propulsion

Very good for swimming

Bilateral symmetry

Well- developed coelom

Ventral heart

Complete digestive tract

Endoskeleton (bone or cartilage or both)

# data[].attributes.body
Sea stars

most have five arms that radiate from a disk

hudreds of tube feet protude from the oral surface

can move in any direction

aboral surface is often covered with spines modified into minute, pincer-like organs called pedicellariae

Brittle stars

very flexible

lack an anus

tube feet lack suckers

the gonads and other internal organs of brittle stars are contained in the central disk and do not extend into the arms

Sea urchins

endoskeleton forms a round, rigid, shell-like test with movable spines

The mouthparts are unique, 5-radiate (of course!), known as Aristotle’s Lantern.  This involves 5 continually growing chisel teeth, each with 8 supporting skeletal pieces.  This gives the teeth remarkable versatility in their action

Sea cucumbers

superficially worm-like

do not have spines and appear to lack radial symmetry

elongated along the oral-aboral axis

lies on one side, where the five rows of tube feet are concentrated

are deposit feeders

when disturbed, some discharge sticky, sometimes toxic filaments through the anus to discourage potential predators

Crinoids

suspension feeders that use outstretched, feathery arms to obtain food from the water

# data[].attributes.body
Radial symmetry

exoskeleton of calcium carbonate

water vascular system with tube feet for movement

calcite spicules embedded in the skin, often partly fused

tube feet (podia)

extensions of the canal system

usually emerge through openings in skeletal ossicles

permit exchange of respiratory gases and nitrogenous waste

sensory functions

Water vascular system

complex system of water filled canals

extensions of tubed feet

Oral opening or mouth normally faced downward with moveable oral spines around it

Have "no brain" but instead a nerve ring that encircles the mouth, radial nerves that extend to each limb, and a nerve net that coordinates the function of the body wall

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Cells of rotifera grow larger as the organism ages, but they don't increase in number. Female rotifers are larger than males and are able to reproduce asexually without the help of a mate. However, a male and female can produce offspring through sexual reproduction during direct physical contact.

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Polyplacophora

Chitons are dioecious, which means there are both male and female chitons. The male releases the sperm into the water and it is taken up by the females.

Gastropods

In many marine gastropods there are separate sexes (male and female); most terrestrial gastropods however are hermaphrodites. Courtship is a part of the behaviour of mating gastropods.

Cephalopods

Cephalopods reproduce only once.  When females reach sexual maturity they will mate and lay their eggs.

Bilvalves

Most marine bivalves free spawn, releasing sperm and eggs into the water where fertilization occurs; the larvae then mature as plankton

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The sexes are separate in most echinoderms. The gonads, 5, 10, or more in numbers, are usually located in the body cavity and open to the outside by way of a duct. They shed sperm or eggs directly into the water. Gametes do no survive long in the water, so in many species individuals spawn all at once to ensure fertilization. Development of the fertilized egg proceeds in the plankton and typically results in a ciliated larva characteristic of each group. Echinoderm larvae are bilaterally symmetrical and it is not until metamorphosis that radial symmetry develops. Some echinoderms do not have planktonic larvae and brood their offspring in special pouches or under the body. Some sea stars, brittle stars, and sea cucumbers regularly produce asexually by the separation of the central disk or body into two pieces. The resulting halves then grow into complete individuals. Regeneration, the ability to grow lost or damaged body parts, is highly developed in echinoderms. Sea stars, brittle stars, and crinoids regenerate lost arms. Some sea stars can have a severed arm and have it grow into a new individual. In some sea stars however, only arms that include portions of the central disk can regenerate.

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Sea Squirt (urochordata)
The sea squirt spawns by releasing eggs and sperm into the water. After about three days, eggs develop into free-swimming, tadpole-like larvae.
Lancelet (Cephalochordata)
Lancelet sexes are separate, and asexual reproduction does not occur. Eggs and sperm are shed directly into the water, where fertilization occurs. The early stages of development strikingly resemble those of both tunicates and vertebrates.

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Cnidarians all have a life cycle of two forms. One is a free-swimming jellyfish medusa stage and the second is an attached polyp stage. Cnidarians have a radially symmetrical body shape with a mouth surrounded by rings of tentacles.

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Encrusting sponges form thin, sometimes brightly colored growths on rocks or dead coral

# data[].attributes.body
Coralline sponges have a soft body that covers a hard, often massive skeleton made of calcium carbonate.

# data[].attributes.body
glass (siliceous) sponges made of four (or six) pointed siliceous spicules. They are found in Antarctica.

# data[].attributes.body
Siliceous sponges and are the most diverse in the phylum Porifera (90% of all species of sponges). They are found in tropical waters

# data[].attributes.body
Sponges whose spicules are made out of calcium carbonate in the form of calcite or aragonite. Mainly in temperate regions, Calcarea sponges are usually found in shallower, sheltered waters less than 1000 m. In tropical regions they are associated with coral reefs.

# data[].attributes.body
Sponges with a soft body that covers a hard, often massive skeleton made of calcium carbonate, either aragonite or calcite.  They are coralline or tropical reef sponges

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Porifera are more formally known as sponges, who are sessile organisms (meaning they live on the sea floor). They have porous bodies which means they don’t have any specialized tissues! Porifera are characterized by special cells called coanocytes. A sponge's body plan consists of a large sac with holes along its sides. In these holes, water is drawn into the large central cavity known as the spongocoel, where it is released through the large hole on the top called the osculum. Sponges consume food and nutrients by forming currents and trapping food by flagellated cells called choanocytes, which are lined up in the spongocoel. Amebocytes travel throughout the inner cells transporting nutrients and form spicules for support.

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Sponges are hermaphrodites. Male and female gametes are formed, while male gametes disperse into the water column, female gametes remain inside the organisms. When male gametes fertilize with female gametes, a larvae is created. Adult sponges have the choice to procreate sexually or asexually. Sponges live from the poles to the tropics, but the largest number of species inhabits shallow tropical waters.

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The wonderful world of the sea sponges- The Abyss- BBc Wildlife

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Want more natural history and wildlife videos? Visit the official BBC Earth channel: http://bit.ly/BBCEarthWW BBC Earth The BBC Earth YouTube channel is home to over 50 years-worth of the best animal videos from the BBC archive. With three new videos released every week there's something for all nature loves from astounding animal behaviour to beautiful imagery.

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The life cycle includes a minute polyp as in scyphozoans.

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They're microscopic!

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Octopuses are extremely intelligent invertebrates and renowned escape artists. Check out Inky's story here - http://franimals.org/?p=1756&preview=true

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Crustacean

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In a laboratory in Sweden, a dark experiment is taking place... just kidding. We played around with a sea squirt, also known as sea pork.

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Sea Cucumber Fights with Guts (Literally) | World's Weirdest

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Chelicerates

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Acoelomate: No body cavity is present between the digestive tract and outer body wall

(Ex. Platyhelminthes)

Pseudocoelomate: A fluid-filled body cavity separates the digestive tract and outer body wall.

(Ex. Round worms, Rotifers)

Coelomate: Fluid-filled body cavity completely lined with mesoderm.

(Ex. Everything else.)
Blastopore: first opening of the archenteron
A. Protostome:  (mouth first)

nSpiral cleavage: planes of cell division are diagonal to the vertical axis.

nDeterminate cleavage: The development fate of each cell is established early.

(Ex. Mollusca, Annelida)

B. Deuterostome: (mouth second)

nRadial Cleavage: cell division that is parallel or perpendicular to the axis.

nIndeterminate cleavage: The tendency of a cell to be able to form an embryo if broken off.

(Ex. Echinodermata, Chordata)

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Protostomes develop spiral cleavage and deuterostomes develop radial cleavage