THE CRAYFISH 225 



delicate. Oxygen, dissolved in the water, passes into the blood by os- 

 mosis, during which process the blood loses some carbon dioxide. The 

 gills are kept from drying by being placed in a nearly closed chamber, 

 which is further adapted to its function by means of the row of tiny hairs 

 which border the lower edge of the carapace. Thus crayfish may live for 

 long periods away from water. 



Circulation. The circulation of blood in the crayfish takes place in 

 a system of thin-\\all <1. tiabby vessels which are open in places, allowing 

 the blood to come in direct contact with the tissues to which it flows. 

 The heart lies on the dorsal side of the body, inclosed in a delicate bag, 

 into which all the blood in the body eventually finds its way during its 

 circulation. 



Cniyfi-h with thr left h:;lf of the body structures removed : a, intestines; 6, ventral 

 artery; c, hrain : -, heart; et, gastric teeth; i, oviduct; /, digestive gland: 

 m, muscles ; n, green gland (kidney) ; o, ovary ; p, pyloric stomach ; r, nerve cords ; 

 .s, cardiac stomach ; st, mouth ; u, telson ; w, openings of veins into the peri- 

 cardial sinus. Natural size. (Davison, Zoology.) 



Digestion. Food which is not ground up into pieces small enough 

 for the purpose of digestion is still further masticated by means of three 

 teeth, strong projections, one placed on the mid-line and two on the side 

 walls of the stomach. The exoskeleton of the crayfish extends down 

 into the stomach, thus forming the gastric mill just described. 



The stomach is divided into anterior and posterior parts separated 

 from each other by a constriction. The posterior part is lined with tiny 

 projections from the wall which make it act as a strainer for the food 

 passing through. Thus the larger particles of food are kept in the 

 anterior end of the stomach. Opening into the posterior end of the 

 stomach are two large digestive glands which further prepare the food for 

 absorption through the walls of the intestine. Once in the blood, the 

 fluid food is circulated through the body to the tissues which need it. 



Nervous System. The internal nervous system of a crayfish con- 

 sists of a series of collections of nerve cells (ganglia) connected by means 

 of a double line of nerves. Posterior to the gullet, this chain of ganglia is 

 found on the ventral side of the body, near the body wall. It then en- 

 HUNT. ES. BIO. 15 



