GENERAL ANATOMY OF THE SQUID. 339 



2. On each side of it is a lateral chamber (Fig. 176, i), 

 open posteriorly but closed anteriorly, and entirely sepa- 

 rated from the cavity of the ventral division. 



It will be seen that when the walls of the mantle cham- 

 ber contract to expel the contained water, any water which 

 is driven into these lateral chambers will simply force their 

 free posterior margins out against the mantle, thus forming 

 a valve to prevent the water from passing out. 



The only exit will then be through the ventral chamber ; 

 and during life, the stream of water which is thus driven 

 through the ventral siphon at each respiration, is the prin- 

 cipal means of locomotion. 



The superficial appearance of the contents of the mantle 

 chamber varies considerably, according to the sex of the 

 specimen. When the mantle of a male specimen is laid 

 open it presents the appearance shown in Fig. 177, but 

 most of the structures shown in this figure are, in the 

 female, covered up by the large, hemispherical, white, finely 

 striated nidamental yhoitlx. When these are removed the 

 parts under them are much like those of the male, but the 

 student should, if possible, select a male specimen for 

 studying the general anatomy. 



i. In the male specimen, notice, in the middle line, just 

 behind the siphon, the rectum (Fig. 177, q), which is 

 bound down onto the- other viscera by a mesenteric fold. 

 At its anterior end notice the anus (/), guarded by a pair 

 of ear-like valves. Dorsal to the intestine, but projecting 

 beyond it so as to be visible on each side of it in a ventral 

 view, notice the ink bag (Fig. 177, m). 



j. Running forward from it on the inner surface of the 

 intestine, notice the ink duct, which opens into the siphon, 

 behind the tip of the rectum. 



k. In the male, notice on the right side of the intestine 



