io6 PHYLUM MOLLUSC A 



Observe again the visceral mass ; it is covered by a thin, trans- 

 parent membrane, the body wall, the extreme thinness of which is 

 correlated with the thickness of the mantle which covers it. If 

 the animal is a female that fact may be known by the presence of 

 two very large, transversely striated bodies, called the nidamental 

 glands, which He near the center of the body, and are a part of the 

 reproductive system. Carefully remove these in order to expose 

 the organs beneath. If the animal is a male (and the student 

 should obtain a male if possible) it can be recognized by the 

 absence of nidamental glands and also by the presence of the 

 testis, a large, white, tubular organ which lies near the median line 

 toward the hinder end of the animal. In the female the ovary, 

 which occupies a similar position, is often very full of the gran- 

 ular ova. 



Notice in the mantle cavity the pair of plumose gills to the 

 right and the left of the visceral mass, each attached to the inner 

 surface of the mantle by a mesentery. Between the retractor 

 muscles of the siphon and extending from the base of the gills 

 forward to the siphon is the rectum, which terminates in the anus, 

 with its two projecting valves. Find the valves. Beneath the 

 rectum is the ink bag, and both are attached to the organs be- 

 neath them by a mesentery. The ink bag communicates with the 

 rectum by means of a duct which joins it near the anus ; this duct 

 may be found by sHtting the rectum for a short distance back of 

 the anus, when the small opening may be made to appear by 

 squeezing the ink bag and forcing the ink into the rectum. The 

 ink, together with the fecal matter from the intestine and other 

 waste products, is voided into the sea water through the siphon ; 

 its function is to cloud the water and thus hide the animal from 

 its enemies. In the male animal notice the long, tubular penis to 

 the right of the rectum (the animal's left) ; if the animal is a female, 

 the thick-walled oviduct will be seen in a corresponding position. 



At the base of each gill note a round, disk-like body ; this is a 

 branchial heart, from which blood is sent into the gills ; near each 

 branchial heart toward the median line and running forward 

 alongside the rectum is an elongate, transparent structure, the 

 kidney. The position of the kidneys may be determined by the 



