132 GENERAL ZOOLOGY 



are homologous parts because they are structurally similar. 

 The same is true, to take another case, of the swimmerets of 

 the third, fourth, and fifth abdominal somites ; but whereas the 

 antenna and the legs are homologous, they are not analogous, 

 because their functions are different. Ordinarily, zoologists 

 use the word " analogous " in a little different way from that 

 suggested above. Instead of being applied to structures in 

 the same animal, it is most commonly applied to organs of 

 similar function in different animals ; for example, the wings 

 of insects and birds, organs which have the same use but 

 are not homologous. 



The Digestive System. The mouth (Fig. 67, 17) of the cray- 

 fish is located between the two mandibles. Reference to 

 Fig. 67, 18, will give an idea of the length of the oesoph- 

 agus and the position and relative size of the stomach 

 (Fig. 67, 19). The latter has two more or less clearly marked 

 ' regions, the anterior enlarged region and the posterior 

 funnel-shaped space. As the food passes into the anterior 

 portion the unbroken bits are caught between the contig- 

 uous grinding surfaces of three hard processes (Fig. 67, 20) 

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

 one in the median dorsal line and two at the sides together 

 constitute the " gastric mill." Muscles attached to them and 

 to the inner surface of the carapace, by contracting, perform 

 the operation of grinding. The posterior part of the stomach 

 is filled with slender filaments which extend from the wall 

 out into the cavity (Fig. 67, 21). These filaments prevent the 

 unbroken particles of food from passing through into the 

 intestine, allowing only the thoroughly ground-up food to 

 do so. Absorption does not occur in either division of the 

 stomach /but in the intestine (Fig. 67, 24), which extends 

 straight 'from the stomach to the ventral surface of the tel- 

 son. A pair of digestive glands (Fig. 67, 23) lie one on either 

 side of the stomach and intestine. They open by tubes into 



