\ 



CHAPTER XIII 



CRUSTACEANS 



Crustacea 



Crustaceans include the lobsters, crabs, and shrimps which 

 are well known to every one in one way or another. But for 

 each of these familiar ones there are thousands that are en- 

 tirely unknown to most persons. They belong to the Arth- 

 ropoda, the phylum of animals which also includes myriapods, 

 insects, and the spiders and mites. Many of these are land 

 animals, such are the millipeds and centipeds, the spiders, and 

 nearly all of the adult insects. But crustaceans have remained 

 predominantly aquatic, and their habits and shapes are inter- 

 locked with the necessities of water life. Fresh water con- 

 tains only a small part of them ; the majority live in the sea. 



Form and habits of crustaceans in general. — With their 

 man}'' differences all crustaceans still have a similar pattern of 

 body. They have an outer shell or exoskeleton, which may 

 be thick and limy as in crayfishes or delicately transparent as 

 in Daphnia (p. 164) and many other smaller crustaceans. This 

 is the hardened secretion of skin cells, and is molted and re- 

 newed as the animal grows, or as it changes from its young to 

 adult form. 



Crustaceans seldom have anything like a neck region, and in 

 most of them the head is so fused to the thorax that it cannot 

 be moved at all. Like all other arthropods they have jointed 

 bodies and jointed appendages, antennae, legs and even 

 mouth parts being segmented. In the larger forms like the 



158 



