218 



ZOOLOGY 



The mouth parts of a crayfish resting in the aquarium are observed to be 

 constantly in motion, despite the fact that no food is present. If the crayfish 

 is taken out of the water and held with the ventral surface upmost, a little 

 carmine (dissolved in water) may be dropped on the lower surface of the 

 animal. This carmine runs down under the carapace. If now the animal 

 is held in water in the same position, the carmine will appear from both sides 

 of the mouth, seemingly propelled by something which causes it to emerge 

 in little puffs. If we remove the maxillipeds and maxillse from a dead 

 specimen, we find a groove leading back from each side of the mouth to a 

 cavity of considerable size on each side of the body under the carapace. 

 This is the gill chamber. It contains the gills, the organs which take oxygen 

 out of the water. The second maxillae are prolonged down into the groove 

 to serve as bailers or scoops. By rapid action of this organ a current of 

 water is maintained which passes over the gills. 



The gills are outside of the body, although protected by the carapace. If 

 the carapace is partly removed on one side, they will be found, looking some- 

 what like white feathers. The blood of the crayfish passes by a series of 

 vessels into the long axis of the gill; in this organ the blood tubes divide into 

 very minute tubes, the walls of which are extremely delicate. Oxygen, dis- 

 solved in the water, passes into the blood by osmosis, during which process 

 the blood loses some carbon dioxide. Notice that 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. 



Crayfish with the left half of the body structures removed; a, intestine; 6, ventral artery; 

 c, brain; e, heart; et, gastric teeth; i, oviduct; Z, digestive gland; m, muscles; n, green 

 gland (kidney); o, ovary; p, pyloric stomach; r, nerve cords; s, cardiac stomach; 

 si, mouth; w, telson; w, openings of veins into the pericardial sinus. Twice natural size. 

 Davison, Zoology. 



The laboratory exercise should conclude with a drawing of the animal 

 from the side, about natural size, with part of the carapace cut away to 

 show the gills. Show as many of the above-mentioned parts as possible. 

 For other useful drawings see Hunter and Valentine, Manual, page 124. 



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

 system of thin-walled, flabby vessels which are open in places, allowing the 

 blood to come in direct contact with the tissues to w^hich 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. 



