40 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



Structure. Note the same regions as in the crayfish 

 (cephalothorax and abdomen) ; study the points of re- 

 semblance and of difference; indications of segmenta- 

 tion shown by each ; compare the number, position, and 

 structure of the legs ; study the spinnerets, their posi- 

 tion, number, and form ; the mouth parts ; the eyes ; 

 the structure and appendages of the skin ; the openings 

 of the breathing chambers (Figs. 81, 82, 216, 223). 



Dissect alcoholic specimens in a dish of weak alcohol 

 or of water, and note the arrangement of the digestive 

 and nervous systems. 



Mollusca 



Mussel 



The river mussels may be kept in aquariums having 

 two to four inches of sand or mud on the bottom. 



Aquarium Study. Study the movements of the ani- 

 mal ; the opening and closing of the shell ; the position 

 and use of the foot ; of the siphons ; the sensitiveness 

 of the siphonal tentacles ; the incurrent and excurrent 

 streams of water (Fig. 86). 



Structure. Examine a shell, noting the two similar 

 valves ; the hinge and hinge ligament ; the hinge teeth 

 (if present); the "epidermis," lines of growth, and 

 nacre ; the scars left by the muscles (Fig. 296). 



Examine the soft part of the body, the mantle lobes ; 

 the gills; the body and the foot; the palpi and the 

 mouth ; the anal opening ; the adductor, protractor, and 

 retractor muscles. 



Cut through the body lengthwise and trace the course 

 of the alimentary canal. Endeavor to trace the parts 

 of the nervous system ; the cerebral and the visceral 

 ganglia; the heart; the digestive gland (Figs. 244, 332). 

 Make three or four cross sections from a specimen 



