THE CRAYFISH 325 



between the segments of the abdomen are merely places where the 

 skeleton is thin and flexible, not where it is absent. A similar 

 skeleton composed of chitin without the carbonate of line is found 

 in other classes of the Phylum Arthropoda. Like the cuticle of an 

 annulate (cf. Fig. 145, p. 302) this arthropod skeleton is secreted 

 by the underlying epidermal cells. It is everywhere continuous 

 and it cannot receive additions at its edges with the growth of the 

 animal, as does the shell of a mollusc. Hence, the crayfish, like 

 other arthropods, is confronted with the problem of increasing in 

 size while it is encased in a skeleton that cannot be stretched. 

 Crayfishes outgrow their skeletons as children outgrow their 

 clothes, but they have the advantage that a new skeleton is secreted 

 inside the old one, which is then "molted." Such a process is 

 characteristic of the Arthropoda (cf. Fig. 169, p. 345) 



Appendages. — Even the most superficial examination shows 

 that the appendages are adapted for different functions in different 

 regions of the body. The antennules and antennae are sensory 

 organs; the mandibles, maxillae, and maxillipeds, organs of feed- 

 ing; the chelae, organs used for defense and offense and for the 

 capture of prey; the pereiopods, organs of locomotion. In the 

 female, the abdominal appendages have the specific function of 

 carrying the eggs (Fig. 155). In the male the two anterior pairs 

 of swimmerets are modified as organs for the transfer of sperm to 

 the seminal receptacles of the female. The uropods, together with 

 the telson, function as the powerful tail-fin by which the animal 

 darts backward through the water. There is, therefore, great func- 

 tional diversity among the appendages of the various regions and 

 there seem to be corresponding differences in structure. Yet all 

 these appendages are constructed upon the same plan and are 

 therefore homologous. This, of course, is but an expression of the 

 metamerism, whereby the parts of each somite are formed accord- 

 ing to a general plan which is modified in relation to its particular 

 functions. Resemblances of this nature between the parts of a 

 single animal are called serial homologies, as distinguished from 

 the homologies between the corresponding parts of different 

 animals. Thus, the fore limbs of all vertebrates are homologous, 

 whereas the vertebrae, the ribs, and the two pairs of appendages 

 of any vertebrate animal are serially homologous. The appendages 

 of the crayfish and similar Crustacea have become a classic exam- 

 ple of this form of homology, because there is no more convenient 



