312 



ZOOLOGY. 



secrete the main digestive fluid, which has a similar 

 universal digesting power to the rabbit's pancreatic juice; 

 they also appear to absorb the digested food, and store up 

 both oil and glycogen as reserves against the starving 

 period that follows a moult. 



16. The Proctodseum (hind-gut, or intestine) is long 

 and straight. If it be true that all absorption takes place 

 in the mesenteron or digestive gland, then the proctodseum 

 can scarcely be said to have any function. It is lined by a 



Fig. 150. HEAKT AND ARTERIES OF ASTAOUS. 

 Side view. (After Howes.) 



cuticle thrown into longitudinal ridges which run in a very 

 slow spiral. 



16. Vascular System Heart. The crayfish has a 

 well-defined heart, which, like that of the mussel, is systemic. 

 In its structure, however, it differs from that of any other 

 of our types. It lies near the dorsal side of the thorax, 

 above and behind the mesenteron. It consists of a single 

 muscular chamber, the walls of which are perforated by 

 three pairs (dorsal, lateral, and ventral) of openings the 

 ostia (figs. 156 and 157). The cavity in which the heart 

 lies is not a coelom, as might easily be supposed from dis- 

 section : it is a blood-sinus. When the heart dilates (i.e. 

 at its diastole) blood rushes in from this pericardial sinus 

 through the ostia ; but these are valved so that when the 

 heart contracts (systole) the blood cannot return, but is 



