170 MOLLUSCA. CEPHALOPODA. 



in the mantle-cavity, in some forms there is one pair, in 

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

 the mantle-cavity, and passes out through the funnel, by 

 means of the contraction of the walls of the mantle-cavity. 

 In the forms belonging to the order Dibranchiata, a gland 

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

 is present ; the duct from this opens together 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 ; 

 it gives off in front an artery to the head, and one behind 

 to the viscera. The nervous system is remarkable in that 

 the three pairs of 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 enclosed in a 

 cartilaginous box and gives off nerves to the arms, viscera, 

 etc. The sexes are always separate, and the genital ducts 

 open into the mantle-cavity. The shell may be external 

 or internal (a), in the latter case it is usually placed in a 

 sac in the mantle on the dorsal side ; in some forms it is 

 absent. 



