112 UNDERWATER GUIDE TO MARINE LIFE 



mandibles, maxillae, or maxillipeds, claw-bearing "arms," walking legs, swim- 

 merets, and tail "fins" on the abdomen. It is the character and number of the 

 appendages that form the basis of classification of arthropods. 



The skeleton of arthropods is well developed and typically formed of a 

 complex substance called "chitin," which sometimes is strongly calcified (crabs 

 and lobsters). 



Unlike the skeletons of vertebrates, however, the skeleton of arthropods 

 covers the body rather than forming an internal structure. Also unlike vertebrate 

 skeletons, the external skeleton does not grow but must be periodically shed as 

 the animal grows inside, the new skeleton always being a little larger than 

 the old. 



The nervous system and the eyes of arthropods are well developed, allowing 

 interesting and complex behavior. The eyes are compound, that is, are composed 

 of several individual small eyes grouped together yet acting as a unit. They are 

 especially well suited for detecting movement since an object produces an 

 image first on one facet of the eye, then another in succession as it moves 

 across the field of vision. 



For the first time in the review of invertebrate phyla, a separate respiratory 

 system is present. This may take several forms— either gills, book "lungs," or 

 tubelike tracheids— but whatever the form, it allows a more efficient respiration 

 than that of previous phyla, which picked up their oxygen through absorption 

 through the surface of the animal. This advance, in turn, allows larger size, 

 higher metabolism, and increased activity. 



For all these reasons the arthropods are well-protected, active animals which 

 have complex behavior. The crustaceans form the most numerous group of 

 arthropods in the sea. 



Crustaceans: Class Crustacea— Figures 37 and 38 



All crustaceans have three rather definite parts of the body— head, thorax or 

 trunk, and abdomen. Sometimes the head and thorax are united into a single 

 unit, as they are in crabs and lobsters, and covered by a single "shell" called 

 the "carapace." Crustaceans have a larval stage called a "nauplius" Qfig. 3i7^, 

 which is common in plankton and is often an extremely bizarre little animal. 

 It is by means of larvae that many crustaceans are distributed through the seas. 



The primitive crustaceans are filter-feeders which sift plankton from sea water. 

 More advanced species are carnivores or scavengers. In fact, the crabs and lobsters 

 are the vultures of the sea, eating literally any organic material, living or dead, 

 that they can lay a claw on. 



Some crustaceans have good powers of defense in their very strong claws. 

 Others are protectively colored or shaped to mimic rocks or seaweeds. Still others 

 can change color so as to match the lightness or darkness of the bottom. Many 

 of the highly developed crustaceans have a remarkable defense involving 

 intentional loss of a limb or autotomy. If a limb is seized bv an enemv, the 

 crustacean snaps it off, then retreats, later growing another limb. 



This class occupies a vital place in the economy of the sea. Many of its 

 species form the major food source of m.any animals. Notable among the many 

 examples is the dependence of the baleen whales on the Euphausian decapods 



