188 STRUCTURE OF THE MOLLUSCA. 



the bottom of the bag formed by the mantle. The water 

 which issues from this cavity escapes through a funnel 

 placed at the lower part of the neck, and also serving for 

 the expulsion of the excrements. There are two branchiae, 

 and at the base of each of these organs is a kind of pulmonic 

 heart, and at the commencement of the great artery of the 

 body an aortic heart, which is composed of a ventricle only. 

 Their stomach is very complex. They secrete a peculiar 

 fluid, of a very dark colour, called ink, which they can expel 

 at will, and which they are said to employ to tinge the 

 water around them, when they wish to conceal themselves. 



These Cephalopoda are extremely voracious, and as they 

 are possessed of activity and great strength, and have nu- 

 merous means of seizing their prey, they are very destructive 

 to fishes and Crustacea. Several are remarkable for the 

 manner in which their skin is covered with variously coloured 

 spots, which alternately appear and disappear. To this group 

 belong the Cuttle-fishes, Argonauts, and others. 



The Octopi have eight nearly equal arms, of great length 

 and vigour, by means of which they clasp their prey. Some 

 authors think that china-ink is made of the black fluid of 

 a species of cuttle-fish, but others are of a different opinion. 



The Argonauts are very similar to the cuttle-fishes, but 

 have two of their arms much widened toward the end, and 

 live in a large convoluted shell, remarkable for its delicate 

 texture and beauty. Some naturalists think that they do 

 not themselves form these shells, but are merely parasites 

 which take possession of them, after the unknown inhabi- 

 tants have been destroyed, in the same manner as the hermit 

 crab lodges in other shells. 



The Cuttle-fishes, fig. 1 , differ from the octopi and argo- 

 nauts in having their body more elongated and furnished with 

 two fins, or, fig. \,bh, fleshy expansions projecting on either 

 side longitudinally ; and ten arms, of which two are much 

 longer than the rest. They have also in the interior of their 

 body a kind of oval shell, composed of a multitude of thin 

 laminae of calcareous matter, and which is employed in the 

 arts, under the name of cuttle or cuttle-bone. 



