THE CRAYFISH 



151 



branched legs after hatching and only later in development 

 loses one branch. The remaining appendages are too com- 

 plicated to be studied here in detail ; but the early stages 

 in the embryo show that all of them, whatever their adult 

 structure, begin their existence as two-branched appendages 

 similar to the swimmerets. 



When two or more organs or structures are constructed 

 upon the same plan and develop in the young in the same 



Gill-bailer 



on second maxilla-. 



First maxilliped -) 



Second maxilliped —? 

 with first gill 



f^^Last gill 



•Sixth gill 



Fig. 85. Gill chamber and gills of crayfish. (Slightly enlarged) 



way, they are said to be homologous. Since all the append- 

 ages of the crayfish are of the two-branched type, they fur- 

 nish a good example of homology. 



The Digestive System. The mouth (Fig. 83) of the cray- 

 fish is located between the two mandibles. Reference to 

 Fig. 83 will give an idea of the length of the esophagus and 

 the position and relative size of the stomach. The latter has 

 two more or less clearly marked regions, — the anterior, en- 

 larged region and the posterior, funnel-shaped space. As the 

 food passes into the anterior portion the unbroken bits are 

 caught between the grinding surfaces of three hard processes 

 extending from the stomach wall. These three "teeth" — 

 one in the median dorsal line and two at the sides — to- 

 gether constitute the "gastric mill." Muscles attached to 

 them and to the inner surface of the carapace, by contract- 



