GENERAL ZOOLOGY 



legs, or pereiopods; all these are thoracic appendages. The abdomen bears 

 five pairs of delicate, paddle-like appendages, the swimmerets, which function 

 in maintaining water circulation, and to which, in the female, the zygotes are 

 attached at the breeding season. In the male the two anterior pairs of ab- 

 dominal appendages are strongly modified as copulatory organs, whereas in 

 the female the first pair is very small. In both sexes the most posterior 

 appendages, borne laterally upon the last abdominal segment, are the fan-like 

 uropods. A median, flap-like outgrowth of the last segment, termed the telson, 

 bears the anus upon its ventral face. The telson is not a segment, and as it is 

 neither paired nor jointed, it is not considered an appendage. The uropods 

 and telson together form the broad tail fin used in swimming. 



There are thus 19 pairs of appendages; 5 pairs belong to the head, 8 to the 

 thorax, and 6 to the abdomen. If we assume that each somite bears a single 

 pair of appendages, there are 19 somites in the body. This count is confirmed 

 by the appearance, during the course of development, of 19 pairs of ganglia 

 in the central nervous system, although not all these ganglia are distinct in 

 the adult animal. 



Comparing the structure of the appendages, we find that all are based on a 

 similar plan, although some are so greatly modified that the homologies are 

 not clearly recognizable until the developmental stages are examined. The 

 simplest appendages are the swimmerets, which in the adult show the funda- 

 mental plan of structure: a basal protopodite with two segments, bearing at 

 its distal end two branches, a lateral exopodite and a medial endopodite. In 

 the pereiopods the protopodite bears in the adult only one distal branch, the 

 endopodite, which is divided into five segments. Exopodites are present on 

 these appendages until a late stage of development but are lost before the 

 animal reaches adulthood. In the maxillipeds the three fundamental divi- 

 sions, protopodite, exopodite, and endopodite, again appear. In the maxillae 

 and mandibles it is necessary to refer to the developmental stages to deter- 

 mine homologies. The mandible, for instance, bears an exopodite during 

 development but in the adult consists of only protopodite and endopodite. 

 The antennae show the three fundamental parts, with the endopodite greatly 

 elongated. The antennules are similarly divided into a basal segment bear- 

 ing two distal branches, here both elongated, but their homology with the 

 other appendages is uncertain. The part of the head bearing the eyes and the 

 antennules probably had an origin different from that of the somites, and 

 hence these sensory outgrowths of the head may not be homologous with the 

 segmental appendages. 



The basic structural similarity of all the true segmental appendages of the 

 crayfish is interpreted as indicating that at some stage in the remote ancestry 

 of these forms, the somites all bore similar, simple, biramous appendages, 

 possibly resembling the swimmerets of the modern crayfish. As regional 

 specialization of the body developed, the appendages underwent diflferential 

 modification in adaptation to the more efficient performance of specific opera- 

 tions. The mouth parts, for example, are clearly adapted in a variety of ways 



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