246 MOLLUSCA. 



CLASS I. 

 CEPHALOPODA. 



Their mantle unites under the body, forming a muscular sac 

 which envelopes all the viscera. In several, its sides are extended 

 into fleshy fins. The head projects from the opening of the sac; it 

 is rounded, furnished with two large eyes, and crowned with longer 

 or shorter conical and fleshy arms or feet, capable of being flexed 

 in every direction, and extremely vigorous, the surface of which is 

 armed with suckers or cups, which enable them to adhere with great 

 tenacity to every body they embrace. These feet are their instru- 

 ments of prehension, natation, and walking. They swim with the 

 head backwards, and crawl in all directions with the head beneath 

 and the body above. 



The Cephalopoda have two branchiae within the sac, one on each 

 side, resembling a highly complicated fern leaf; the great vena cava, 

 having arrived between them, divides into two branches, which pour 

 their contents into two fleshy ventricles, each of which is placed at 

 the base of the branchia3 on its own side, and propels the blood into it. 



Respiration is effected by the water which flows into the sac and 

 issues through a funnel placed at its opening. 



Between the base of the feet we find the mouth armed with two 

 stout horny jaws resembling the beak of a parrot. 



These animals are remarkable for a peculiar and intensely black 

 excretion, with which they darken the surrounding water when they 

 wish to conceal themselves. It is produced by a gland, and held in 

 reserve by a sac, variously situated, according to the species. 



Their brain, which is contained in a cartilaginous cavity of the 

 head, gives off a cord on each side which produces a large ganglion 

 in each orbit, whence are derived innumerable optic filaments; the 

 eye consists of several membranes, and is covered by the skin which 

 becomes diaphanous in that particular spot, sometimes forming 

 folds which supply the want of eyelids. The ear is merely a slight 

 cavity, on each side near the brain, without semicircular canals or 



