LOLIGO PEALII. 93 



siphon. These are the retractor muscles of the siphon. Be- 

 neath them are the similar retractor muscles of the head. 

 Explain the function in each case. How could the former pair 

 operate to affect the direction of movement in swimming ? 



e) Running forward almost to the rear opening of the siphon, midway 



between the retractor muscles of the latter, is the posterior portion 

 of the alimentary canal rectum ending in the anus. Note the 

 two flap-like rectal-valves bordering the anus. What is their 

 probable function ? It is evident, then, that the alimentary canal, 

 which begins with the mouth, has doubled back on itself and that 

 this arrangement will permit the foecal matter to be swept out with 

 the current of sea water ejected from the siphon. Compare this 

 arrangement with that of the mussel. 



f) Beneath the rectum, and bulging out on each side of it, is the ink bag. 



It opens by a small duct into the rectum, as can be demonstrated 

 in another specimen by slitting the rectum for a short distance 

 back from the anus and gently pressing the ink bag to force out 

 the contents. If you have a chance to study the living animal prod 

 it with a pencil and note results. What purpose is served by thus 

 clouding the surrounding sea water with ink ? 



g) Ink bag and rectum are attached to the other organs by strips or 



films of connective tissue called mesenteries. Look for similar 



attachments elsewhere, 

 h) Prominent on each side of the group of organs in the mantle cavity 



is a long plume-like structure, the gill. How does its position and 



its structure adapt it to its function? 

 i) Between the left gill (on your right) and the rectum will be seen the 



tubular penis, a male organ of reproduction. In the female a 



portion of the oviduct will be similarly located, 

 j) A thin transparent membrane, the true body wall, envelops the 



visceral mass. Account for its being so frail, 

 k) Push aside the head and note a pair of large nerve centers, the 



stellate ganglia, with their short nerves radiating to the mantle. 

 1) Running from the base of each gill to the mantle is a branchial 



retractor muscle. 



