CEPHALOPODA. 823 



respiratory organs are in the form of plume-like gills placed sym- 

 metrically on the sides of the body within the pallial sac. The 

 Cuttle-fishes (Dibranchiate Cephalopods) have two gills, one on 

 each side, while the Pearly Nautilus and its allies (Tetrabranchiate 

 Cephalopods) have four gills, two on each side. The currents of 

 water needed in respiration are maintained by the alternate contrac- 

 tions and expansions of the muscular walls of the mantle-sac. In 

 each expansion the water finds its way into the pallial chamber by 

 the opening between the rim of the mantle and the neck ; and in 

 each contraction it is expelled through the tube of the funnel, which 

 is so constructed as to allow of the egress but to prevent the ingress 

 of the water. 



The nervous system of the Cephalopoda is highly developed, and 

 its central masses form an oesophageal nerve-collar, which is pro- 

 tected by a cartilaginous plate. 



The sexes are in different individuals, and the males and females 

 are commonly more or less unlike externally, the former often having 

 one of the arms specially modified to serve as an intromittent organ. 

 The ducts of the generative glands open at the base of the funnel, 

 and each individual, besides the essential organs of reproduction 

 (testis or ovary), generally possesses accessory glands. The most 

 important of these accessory glands in the females are known as 

 the " nidamental glands," and they secrete a viscid material which 

 unites the eggs together. 



The shell of the Cephalopoda is sometimes external, sometimes 

 internal. The internal skeleton (fig. 737) is known as the "cuttle- 

 bone," " sepiostaire," or "pen" (gladius), and may be either cor- 

 neous or calcareous. In some cases it is rendered complex by the 

 addition of a chambered portion or " phragmacone," which is to be 

 regarded as a visceral skeleton or " splanchnoskeleton." In Spirula 

 (fig. 737, c) the phragmacone is the sole internal skeleton, and is 

 coiled into a spiral, the coils of which lie in one plane, and are near 

 one another, but not in contact. It thus resembles the shell of the 

 Pearly Nautilus, but it is internal, and differs, therefore, in this re- 

 spect from the external shell of the latter, though it is so far ex- 

 ternal that the last chamber lodges part of the viscera. The only 

 living Cephalopods which are provided with an external shell are the 

 Paper Nautilus (Argonauta) and the species of Pearly Nautilus 

 {Nautilus) ; but not only is the structure of the animal different in 

 each of these, but the nature of the shell itself is entirely different. 

 The shell of the Argonaut is involuted, but is not divided into 

 chambers, and it is secreted by the webbed extremities of the two 

 dorsal arms of the female. The arms are bent backwards, so as to 

 allow the animal to live in the shell, but there is in reality no organic 

 connection between the shell and the body of the animal. In fact, 



