324 MANUAL OF ZOOLOGY. 



whilst the remainder is covered with recurved spines. The 

 buccal cavity conducts by an oesophagus — into which salivary 

 glands usually pour their secretion — to a stomach, from which 

 an intestine is continued, with a neural flexure, to open on the 

 ventral surface of the animal at the base of the funnel. In 

 many cases there is also a special gland, called 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; but this, apparatus is entirely wanting in the Tetra- 

 branchiate Cephalopods, where, in consequence of the presence 

 of an external shell, this means of defence is not needed. 



The respiratory organs are in the form of two or four plume- 

 like gills, placed on the sides of the body in a branchial cavity 

 which opens anteriorly on the under surface of the body. At 

 the base of each gill, in the Cuttle-fishes, is a special contrac- 

 tile cavity, whereby the venous blood returned from the 

 body is driven through the branchiae. In addition to these 

 accessory organs — ^the so-called "branchial hearts "—there is 

 a true systemic heart, by which the aerated blood receivgd 

 from the gills is propelled through the body. In the higher 

 Cephalopods a capillary system of vessels intervenes, in most 

 cases at any rate, between the arteries and the veins. The 

 admission of water to the branchije is- effected by the expansion 

 of the mantle so as to allow the entrance of the outer water into 

 the pallial chamber. The mantle then contracts, and the water 

 is forcibly expelled through the funnel, which is provided 

 with a valve permitting the . egress of water, but preventing 

 its ingress. By a repetition of this process, not only is respi- 

 ration effected, but locomotion is simultaneously subserved; 

 the jets of water expelled from the funnel, by their reaction, 

 driving the animal in the opposite direction. 



The nervous system is formed upon essentially the same 

 plan as in the other Mollusca, but it is more concentrated, 

 and the supra-CESophageal or cerebral ganglia are protected by 

 a cartilage, which is to be regarded as a rudimentary cranium. 

 This structure, therefore, presents us with the nearest ap- 

 proach which we have yet met with to the Vertebrate type of 

 organisation. 



The sexes in all the Cephalopoda are in different individuals, 

 and the reproductive process in the Dibranchiate section of 

 the class (Cuttle-fishes) is attended with some very singular 

 phenomena. In this order the ducts of the generative organs 

 open into the pallial chamber, and each individual, besides 

 the essential organs of reproduction (testis or ovary), gene- 



