MOLLUSCA. CEPHALOPODA 259 



posterior (under) surface. The feather-like gills (/)) are 

 placed in the mantle-cavity ; in the Dibranchs (cuttle- 

 fishes, etc.) there is one pair, in Nautilus there are 

 two. A current of water flows in at the sides of the 

 mantle-cavity, and can be forced out through the funnel 

 by means of the contraction of the walls of the mantle- 

 cavity. In the Dibranchiate Cephalopods there is a gland, 

 known as the ink-sac (iv), which secretes a black fluid 

 (sepia) ; the duct from this gland opens with the anus 

 (o) into the mantle-cavity ; the ink is ejected at times 

 and passes out through the funnel, rendering the water 

 cloudy, and by this means facilitating the escape of the 

 animal from its enemies. Just within the mouth there 

 are two jaws (g, h) which have the form of a parrot's beak, 

 and are either horny or calcareous. An odontophore (i) 

 is also present, but the arrangement of the teeth is less 

 variable than in the gasteropods, and is of little value for 

 systematic purposes. 



The heart consists of a median ventricle (q), and of 

 lateral auricles, which are either two or four in number, 

 according as there are two or four gills. The nervous 

 system is remarkable in that the ganglia are placed 

 close together, forming a central mass (I) ; one part is 

 placed above the oesophagus, and is connected by cords 

 with the other part beneath it. This central nervous 

 system is covered by a cartilaginous ring and gives off 

 nerves to the arms, viscera, etc. The sexes of the Cephal- 

 opods are always separate, and show external differences. 

 In some genera there is no shell ; but when present it may 

 be external (fig. 103) or internal (fig. 102, a); in the 

 latter case it is usually placed in a sac in the mantle on 

 the antero-dorsal side. The Cephalopoda are divided into 

 two Orders : — (1) Tetrabranchia, (2) Dibranchia. 



17—2 



