THE ARTHROPOD A 313 



of the crayfish serves to show in gross much that is otherwise difficult 

 to observe in the insects and lends itself well to an illustration of serial 

 Homology and the so-called Savigny's law. The crayfish has not been 

 well studied, then, unless, after completing this chapter, these things are 

 definitely known. 



EXTERNAL APPEARANCE 



The crayfish is found nearly everywhere in this country and Europe ; 

 in the eastern part of the United States Cambarus affinis ( ) 



is prevalent, while Cambarus virilis ( ) is more plentiful 



in the Middle States, and the European specimen found most frequently 

 is the Astacus fluviatilis ( ). There is little differ- 



ence, however, in their external or internal makeup. It will be remem- 



brc 



Fig. 202. The common Crayfish, Astacus fluviatilis, seen from 

 the side. 



abd. Abdomen. ami*. 1. First walking leg. amb. 4. 

 Fourth walking leg. an'. First antenna or antennule. an". 

 Second antenna, be. Branchiostegite. br.c. Branchiocardiac 

 groove, e. Carapace, ch. Chela, cv.g. Cervical groove. e.s. 

 Eye-stalks. g.g. Opening of green gland, mxp. 3. Third 

 maxillipede. rs. Rostrum, sw. Swimmerets. t. Telson. 15. 

 First segment of abdomen. 20. Last segment of abdomen, xx. 

 The last appendage. (After Shipley & MacBride.) 



bered that the segmentation of the frog is found in the spinal column. 

 With the crayfish, however, segmentation can be observed externally 

 Tunning from anterior to posterior end, though there is a peculiar con- 

 dition of fusing of a number of the anterior segments (Fig. 202) which 

 thus form what is known as the cephalothorax ( ). 



As one may observe in the embryological study of the crayfish that 

 each embryonic segment possesses a pair of appendages, it is but neces- 

 sary to count the appendages in an adult arthropod in order to find how 

 many segments have fused in any given region. This is known as 

 Savigny's Law. 



Beginning at the anterior end of the animal we find the first seg- 

 ment having two pairs of long feelers, the longer ones being the antennae 

 ?.nd the shorter the antennules. 



Directly behind these there is a series of modified appendages (Figs. 



202, 203) directly in front of the large mandibles. These cover the mouth 



itself. Two pair of tiny appendages the maxillae lie anterior to the 



mandible, whik three pairs of appendages the maxillipeds lie pos- 



