CRAYFISH 399 



endopodite. Joining the coxopodite (first segment of protopodite) is a 

 sheetlike structure whicli supports a gill and some chitinous threads. 

 The three anterior walking legs possess pinchers or chela which are 

 formed by the terminal segment being set on the side of the second 

 segment. The walking legs are used in locomotion, offense, and de- 

 fense. The three anterior segments of the thorax bear three pairs of 

 biramous maxiUipeds. The parts are quite typical in most respects. 

 Each has an epipodite joining the basipodite and all except the first 

 bear gills. These appendages are used in getting food to the mouth. 



To the segments of the head are attached five pairs of appendages. 

 Just posterior to the mouth and immediately in front of the first 

 maxilliped are two pairs of maxillae, the second of which overlies 

 the first. They are both leaflike and modified. The epipodite and 

 exopodite of the second are fused to form a bladelike bailer or 

 scaphognathite which fits over the gills and by its movement helps 

 circulate the water for respiration. The endopodite is slender, but 

 the protopodite is broad and foliate. The first maxilla is reduced 

 to a leaflike protopodite and small endopodite. The jawlike man- 

 dible at each side of the mouth is composed of hard protopodite 

 with teeth and a fingerlike endopodite, which is tucked under the 

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

 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 antenmdes 

 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 compaTed, show different structure and 

 origin but similar function. During early development each of the 

 appendages is similar to all others. Some become modified with 

 development. Other illustrations 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 structures are found in manj^ ani- 

 mal groups and are used in establishing relationships. It has been 

 suggested that the parapodia of Nereis represent possible forerun- 



