XII. CRAYFISH 



The Crayfish is a common example of a very large and important 

 phylum of segmented Invertebrates, known as the Arthropoda and 

 characterized, as the name indicates, by the presence of jointed 

 appendages. Typically there is a pair of such appendages on 

 every segment, but as will be seen from our studies of various repre- 

 sentatives of this phylum, wide variations may be found in the 

 number of pairs present as well as in the structural and functional 

 features. All these types of appendages are believed to be related 

 to a basic ancestral type and are, therefore, said to be homologous. 



The Arthropoda are triploblastic, bilaterally symmetrical animals 

 with a ventral nerve cord, and it may be said, in general, that 

 various structural features are present which seem to link them 

 rather closely with the Annelida. In fact, one primitive Arthropod 

 genus, Peripatus, is very definitely worm-like in its general struc- 

 tural plan but, nevertheless, has jointed appendages and other 

 Arthropod structures. On the whole, though, it is very evident 

 that the organization of the Arthropoda is more advanced than 

 that of the Annelida. Thus the number of segments is definite 

 and limited in Arthropod species, and typically exhibit a consider- 

 able degree of segmental specialization and fusion to form three 

 body regions, the head, thorax, and abdomen. The development 

 of the anterior end of the body to form a definite head — the 

 process of cephalization — together with a marked concentra- 

 tion of the nervous system and the formation of extremely sensi- 

 tive and highly differentiated regions of it for the reception of 

 external stimuli are all features of great interest and importance. 



A. External Structure 



The Crayfish has an elongated body, generally about six inches 

 in length when fully extended. The entire body is covered by a 

 rigid covering (exoskeleton) which is formed as a secretion by 

 the underlying epithelial cells. In the anterior portion of the 

 body (cephalothorax) the dorsal exoskeleton is unsegmented, 

 and is termed the carapace. It ends in an anterior projection 



99 



