CEPHALOPODA. 189 



the " ink-bag," for the secretion of an inky fluid, which the 

 animal discharges into the water, so as to enable it to escape 

 when menaced or pursued. The duct of the ink-bag opens at 

 the base of the funnel near the anus, but the Pearly Nautilus 

 and the allied fossil forms are without this means of defence, 

 which the presence of an external shell renders unnecessary. 



The respiratory organs are in the form of plume-like gills, 

 placed on the sides of the body in a branchial chamber, which 

 opens in front on the under surface of the body. In almost 

 all the living Cephalopoda there are only two gills, one on 

 each side, and hence this section is known as that of the 

 " Dibranchiata." In the Pearly Nautilus alone there are four 

 giUs, two on each side, hence the name of " Tetrabranchiata" 

 applied to the order of which this is the only living represent- 

 ative. In the Cuttle-fishes, at the base of each gill is a special 

 contractile cavity, called a " branchial heart," by which the 

 venous blood, returned from the body, is driven through the 

 gills. In addition to these branchial hearts there is a true 

 arterial heart, by which the aerated blood received from the 

 gills is driven through the body. The admission of water to 

 the branchiae is effected by the expansion of the mantle, which 

 allows the entrance of the outer water into the mantle-cavity. 

 The mantle then contracts, and the water is forcibly expelled 

 through the funnel, which is often furnished with a valve, al- 

 lovfing the passage of water outward, but preventing its en- 

 trance inward. By a repetition of this process both respira- 

 tion and locomotion are simultaneously effected, for the jets 

 of water expelled from the funnel by their reaction drive the 

 animal in the opposite direction. In this case, therefore, as in 

 many others, the more active the animal is, the more perfectly 

 is the respiratory process carried on. 



The nervous system is formed upon essentially the same 

 plan as in the other Mollusca, but the cerebral ganglia are 

 protected by a cartilage, which is to be regarded as a rudimen- 

 tary skull. This structure, therefore, is a decided approach to 

 the Vertebrate type of organization. 



The sexes in all the Cephalopoda are in different individ- 

 uals, and the reproductive process in the Cuttle-fishes is at- 

 tended with some singular phenomena. The most remarkable 

 point in this connection is the modification of one of the arms 

 of the male Cuttle-fishes, for the purpose of conveying the 

 male element to the female. The details of the modification 

 vary in different species of Cuttle-fish. 



In some species one arm is simply so modified as to be 



