206 INVERTEBRATE ZOOLOGY 



namely, a proximal coxopodite and a distal basipodite. Distally, 

 the basipodite characteristically bears two branches of which 

 the one nearer the median line of the body is termed the endopo- 

 dite and the other the exopodite. Many of the crustacean 

 appendages have become modified so that the biramous con- 

 dition is not observable, as, for example, the walking legs of the 

 crayfish and lobster in which the protopodite bears but a single 

 series of segments to form the leg. A study of the larvae of the 

 lobsters (Fig. 95) gives proof that the distal part of the leg is 

 the endopodite, for in larval lobsters the thoracic legs are dis- 

 tinctly biramous and it is only in later development that the 

 exopodite of these appendages disappears. There are no free 

 larval stages in the crayfishes, for the entire development is com- 

 pleted within the egg, but since other structures of the crayfish 

 homologize so directly with those of the lobster, the leg of the 

 crayfish is likewise considered as being composed of a protopodite 

 and an endopodite. 



While the schizopodal or biramous appendages are character- 

 istic of most crustaceans, there are some in which a more primitive 

 type of appendage is found. In the Phyllopoda, there are leaf- 

 like feet on the thorax (Fig. 97) though the antennae are bira- 

 mous. The phyllopod appendages bear a number of lateral 

 processes called endites projecting from a central axis of podo- 

 meres or foot segments. This type of appendage seems to be 

 more generalized than the biramous type and the latter may 

 have originated as a modification of the foliaceous type. 



The number of body segments is highly variable. The 

 anterior five somites are fused with the prostomium to form the 

 head. This may be united with some or all of the thorax to 

 form a cephalothorax. The abdominal somites are highly 

 variable in number. 



Respiratory and Excretory Organs. — Respiration is usually 

 by means of gills, though in some instances there are no modified 

 structures for respiration, because the entire body surface func- 

 tions in this capacity directly. The gills are very frequently 

 borne within a gill chamber formed by the walls of the carapace 

 and the body wall. Special organs of excretion are the green 

 glands and the so-called shell glands which open on the bases of 

 the second antennae and the second maxillae respectively. 

 These two organs occur together in the larvae, but in adult 

 crustaceans one or the other fails to develop. They agree 



