A Cephalopodous Mo Husk. 71 



Examine one of the larger suckers. Is it sessile or 

 mounted on a peduncle ? Note the horny ring. Is the 

 margin of the ring entire or serrate ? Make a longitudinal 

 section through the sucker and its fleshy piston. 



Draw the side view of the section. 



Internal Anatomy. Carefully open a male specimen, 

 a little to one side of the mid-ventral line, exposing the 

 mantle cavity. Pass a large probe through the opening of 

 the siphon. Do you find the large retractor muscles of the 

 siphon? Trace them to their posterior points of attach- 

 ment. On each side of the siphon note the lateral siphonal 

 blind-sacs. What is their function? Immediately posterior 

 to the siphon note the tube-like rectum. 



The anus, at the most anterior point of the rectum, will 

 be found guarded by two lateral, leaf-like valves. Carefully 

 inject water into the rectum, and note the outline of the 

 organ. 



Immediately dorsal to the rectum is a dark-colored sac, 

 the ink-sac. Slit open the anterior end of the rectum, 

 along its mid-ventral line, and note the point of entrance of 

 the tube from the ink-sac. 



Lying along each side of the mantle cavity, and parallel 

 to the retractors of the siphon, are the gills. How is each 

 gill attached? Do you find, at the base of each, a disc- 

 shaped branchial heart ? The median mantle artery will be 

 noted, as a cord-like tube, extending from a point midway 

 between the branchial hearts, across the mantle cavity to the 

 inner side of the mantle. It marks the anterior limit of a 

 median mesenteric fold that extends from it posteriorly. 



Looking through the transparent tissue that covers the 

 viscera, a pair of underlying urinary organs, or nephridia, 



