PHYLUM ARTHROPOD A — CLASS CRUSTACEA 271 



with teeth ajid a fingerlike endopoclite, which is tucked under the 

 anterior edge of the former. This appendage is used for chewing. 

 In front of these are the antennae which are biramous and are some- 

 times called "feelers." They consist of the protopodite of two 

 parts, a long many-jointed, filamentous endopodite and a relatively 

 short, fan-shaped exopodite. Anterior to these are the antenmiles 

 which are biramous and feelerlike. The exopodite and endopodite 

 are similar in these. 



The principle of homology is excellently illustrated by the ap- 

 pendages of the crayfish. In general, homologous structures are 

 those which have similar structure and similar origin but may have 

 similar or different functions. By way of contrast, analogous struc- 

 tures are those which, when compared, show different structure and 

 origin but similar function. During early development each of the 

 appendages of the crayfish is similar to all others. Some become modi- 

 fied with development. Other illustrations of homologous structures 

 are the human arm and the bird 's wing. In organisms like crayfish 

 where the appendages of successive segments are homologous to each 

 other, the condition is spoken of as serial homology. Homologous 

 stiiTctures are found in many animal groups and are used in establish- 

 ing relationships. It ha.s been suggested that the parapodia of Nereis 

 represent possible forerunners of crustacean legs. They are both typi- 

 cally biramous and both take about the same position on the body, 

 as well as having a similar segmental distribution. There is also con- 

 siderable similarity in their structure. 



Internal Structure 



Beneath the shell-like, chitinous exoskeleton there is a very rep- 

 resentative set of systems. As in most higher animals the segmen- 

 tation is retained in the muscular system, nervous system, and to 

 a degree in the circulatory system. Earlier in the chapter it was 

 pointed out that the coelom is modified as a provision for increased 

 blood sinuses which have occupied much of the space. 



Respiratory System.^ — Under the branchial areas of the carapace 

 may be found the paired, feathery gills held in the gill cavity or 

 branchial chamber. There are three types of gills present here: 

 pleurohranchiae, attached to the sides of the thorax; podohranchiae, 

 arising from the epipodites of the thoracic appendages; and arthro- 



