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Crustacea is the taxonomic group (now treated as a sub phylum due to recent studies)  that is the classification for many arthropods including crabs, shrimp, lobster, crayfish, krill, woodlice, and barnacle. Their size can range from 0.1 mm to 3.8 m of a leg span. Some are parasitic, and use their hosts as a habitat. Others are more terrestrial, which is more common due to the fact they're known to have the largest biomass on the planet. Crustaceans have exoskeletons, and depending on the species, they will moult out of it to grow and then grow a new one. Crustaceans body is categorized into three different parts, the head, the thorax, and the abdomen. It's also important to mention that crustaceans all have open circulatory systems, which means  that the blood is not transferred through veins and arteries, instead it is completely open.

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- They are the largest species of crab
- They have five gills used for respiration
- They 6 legs
- Their heart is neurogenic, meaning that it is controlled by the nervous system
- They have an open circulatory system
https://upload.wikimedia.org/wikipedia/commons/a/a7/Spider_crab.jpg

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- They are born male and change to female as they grow older
- They have five pairs of leg
- They can grow to a length of 9 in.
- They have a life span of 18 months
-  In the postlarval stage, they will migrate to estuaries, and feed and grow until they reach the length of 110-120 mm, then they will go back to the ocean
http://www.fao.org/fileadmin/user_upload/affris/img/wpF3.jpg

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- They can usually be found attached to several species of deep-sea squaloid sharks
- This relationship however, is parasitism as the anelasma is gaining the nutrition
-  They reach a length of 25 mm
- Unlike most barnacle, it does not have a shell
- Instead of a shell, it  has a purple-black mantle that is tough
- It has an opening on the top of their mantle
https://upload.wikimedia.org/wikipedia/commons/thumb/6/62/Anelasma_squalicola_attached_to_Etmopterus_compagnoi_Griffiths.png/800px-Anelasma_squalicola_attached_to_Etmopterus_compagnoi_Griffiths.png

NOTE:  The two black objects are the anelasma, not the shark

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Kingdom- Animalia

Phylum- Arthopoda

Class- Malacostraca
Order- Decapoda
Family- Penaeida
Genus- Fenneropenaeus
Species- F. indicus

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Kingdom- Animalia
Phylum- Arthopoda
Class- Malacostraca
Order- Decapoda
Family- Lithodidae
Genus- Paralithodes
Species- P. camtschaticus

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Kingdom- Animalia
Phylum- Arthopoda
Class- Maxillopoda
Order- Penduculata
Family- Anelasmatidae
Genus- Anelasma
Species- A. squalicola

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Taxonomy can show the relatedness of organisms within a taxonomic group because the different levels serve as classification, and shows the evolutionary history of the organisms

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The three exemplary organisms all have  have exoskeletons made of proteins called chitin and calcium. These proteins provide protection and rigid support for the attachment of the muscles.

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http://www.chemistryexplained.com/elements/images/chel_0001_0001_0_img0070.jpg

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https://upload.wikimedia.org/wikipedia/commons/thumb/7/7e/Haworth_projection_of_chitin.svg/250px-Haworth_projection_of_chitin.svg.png

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All three exemplary organisms have a larvae form right before they are born and for a little bit after they are born. Later days, months, or even years is when they will grow to their adult forms. All larvae go through their stages by growing small exoskeletons for temporary protection, and then moult to grow further towards their adult forms.

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https://www.pinterest.co.uk/pin/453174781229895797/

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https://www.arkive.org/common-prawn/palaemon-serratus/image-A24712.html

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https://seagrant.uaf.edu/research/projects/kingcrab/newsflash/1305akcrrab.html

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This is a fossil of the crab species Pulalius vulgaris. It comes from the eocene period, which was approximately 56 million years ago to 33.9 million years ago. Most fossils of this species is found to be around 3 inches in length, and 1.25 inches in depth
http://www.fossilmall.com/Pangaea/pfossils/pf82/pffossils82.htm

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Museum Fossil Crab Pulalius from Washington Sate

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This is the fossil of the

Solnhofen Mesolimulus Horseshoe Crab, or the common name, the horseshoe crab. This fossil ages back 163 million years ago. But horseshoe crabs were around before dinosaurs, around 445 million years ago, and are still around today. They are more closely related to spiders and scorpions then they are to crustaceans.

http://www.fossilmuseum.net/Fossil_Sites/solnhofen/Mesolimulus-walchi/Mesolimulus-walchi-t.jpg

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This is a fossil of the Aeger tipularis Shrimp, and is dated back to 145 million years ago. Much of their exoskeleton is still intact.
http://www.fossilmuseum.net/Fossil_Sites/solnhofen/Aeger-tipularis-solnhofen/Ep004ct.jpg

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This is the fossil of the Astacus spinorostrinus. It comes from the Jurassic period. It is 3.75 inches in length, and was found at the Lingyuan Formation in China.
http://www.fossilmall.com/Pangaea/pcrust/pcrust28/Astacus_fossil.jpg

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This is a lobster fossil of the species Scyllaridae indet, and is dated back 93-97 million years ago. The size of this fossil 26 mm in length, and was found at Hajoula, Lebanon.
https://lh4.googleusercontent.com/KV5Mm6HgCeCgggenTmQnPHePEW1HHxPRM7rZofTouzCBjEDh91HUwaHshD6O7YdD6pcD5zm8qVcw8FQAj5Lwbgcrpbaf2gIZOw=w240-h180

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This is a shrimp fossil of the species Atrimpos meyeri, and is 150 million years old. It is 3.25 inches in length, and was found in Eichstatt, Germany.
http://www.fossilmall.com/EDCOPE_Enterprises/invertebrates/invert93/atrimpos_shrirmp_fossil.jpg

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All three organisms body consists of exoskeleton, to protect their soft bodies. They also have all have gills for ion transport, acid-base balance, and ammonia excretion.
https://bio.libretexts.org/@api/deki/files/4309/img025.jpg?revision=3

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It can be determined that crustaceans have gone through gradual speciation rather than punctuated equilibrium, because phylogenic trees show that crabs, lobster, etc. came from sea-inhabiting arthopods, like barnacle. Over a long period of time, species of crustaceans started to become more complex and diverse, eventually leading to physical isolation. This led to new species, over a long period of time.

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It can be inferred that crustaceans have gone through divergent evolution, because of their anatomy. The exoskeletons and ultrastructure of the compound eyes from the shrimp, millipedes, and horseshoe crab shows that they all come from a common ancestor.

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Some species of crustaceans choose their mate by their physical characteristic, rather than random selection. For example, brine shrimp females will choose their mate depending on their size and color. Also, some crabs under the genus trapezia will choose their male mates based on size

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There is an interesting example of artificial selection among crustaceans. Some samurai crabs have designs on their shells, that almost resemble a human face. So when fisherman is to pick between eating a normal looking crab or a crab that looks like a human face, there most likely going to throw the human looking crab back into the water. This means that if a samurai crab's shell looks like a human face, they are more likely to survive.

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Darwin's Descent with Modification theory applies to Crustaceans because many females will choose their mates based on size. The bigger the size of the organism, the more intimidating it is to the predator. Descent with Modification shows how Parent crustaceans will pass down their advancements in genes (larger size gene) to their offspring so that they will thrive and reproduce better.

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Directional Selection is when an extreme phenotype is favored over all other phenotypes. Due to the predators of crustaceans, crustaceans will develop defense mechanisms for the predators. One of these include their color. Many crustaceans will flash their colors to intimidate their predators, giving them a greater chance of surviving. So therefore, crustaceans with dull colors will die out, and the extreme phenotype of bright colors will continue to thrive and reproduce.