The Arthropoda 319 



respiratory functions. The eight segments following the head consti- 

 tute the thorax and the seven last segments (counting the rudimentary 

 twentieth segment), the abdomen. 



"Crustaceans being primitively free-swimming aquatic animals, it is 

 their swimming appendages that are least altered by adaptations. The 

 legs are the stoutest of the appendages, and these offer but one branch 

 arising from the basal piece, and that composed of a reduced number of 

 highly differentiated segments. A comparison of a leg with the last 

 maxilliped in the crayfish will show which appendage has been lost and 

 which preserved and specialized. The best clues to interpretation of 

 homologies in any appendage are likely to be found in other adjacent 

 appendages, which, because of proximity, have been subject to somewhat 

 similar influences." 



THE DIGESTIVE SYSTEM 



Crayfish live chiefly on living snails, tadpoles, young insects, and 

 the like, but sometimes eat one another, and may also devour decaying 

 organic matter. They feed at night, being most active at dusk and day- 

 break. The maxillipeds and maxillae hold the food while it is being 

 crushed into small pieces by the mandibles. The food particles pass 

 down the oesophagus into the anterior, cardiac chamber of the stomach, 

 where they are ground up by a number of chitinous ossicles forming the 

 gastric mill (Fig. 205). When fine enough, the food passes through a 

 sieve-like strainer or hair-like setae into the pyloric chamber of the stom- 

 ach ; here it is mixed with a secretion from the digestive glands brought 

 in by the hepatic ducts. The dissolved food is absorbed by the walls of 

 the intestine. Undigested particles pass on into the posterior end of the 

 intestine, where they are gathered together into faeces, and egested 

 through the anus. 



THE CIRCULATORY SYSTEM 



As in the frog, the nourishing fluid, the blood, is pumped by 

 the heart (Figs. 206, 207) through the arterial system to the different 

 parts of the body. The blood of crayfish is generally colorless or pinkish 

 in hue, but on standing, especially if exposed to air, it assumes a bluish 

 color. This is due to haemocyanin, a respiratory protein, which has 

 copper in its nucleus. 



Before moulting, the blood of the crayfish is pink in color, due to a 

 dissolved pigment, tetronerythrin, a lipochrome, which is probably de- 

 posited in the new chitinous covering, since it is present in less quantity 

 in the blood after the complete formation of the new exoskeleton. 



The blood of the crayfish transports food, gases, and wastes, quite 

 as does the blood of the frog. 



The crayfish does not possess a true venous system and the heart. 

 has only a single large cavity. The open spaces in the animal's body 

 through which the blood is returned to the heart are called sinuses. 



