38 COMPARATIVE ANATOMY CHAP. 



Above the head, the mantle fold encircles the whole body. It is short 

 at the sides, but anteriorly it forms a large lobe which is folded back 

 over the shell in the way shown in Fig. 32, p. 22. Posteriorly, the 

 mantle covers a very deep cavity which contains the whole posterior side 

 of the visceral dome. The siphon consists of two entirely distinct 

 lateral lobes (epipodial lobes), whose free edges overlap in such a 

 manner as to form a tube, open above and below. As we shall see 

 later, this siphon is a part of the foot. Deep down in the mantle 

 cavity, two pairs of pinnate gills a lower and an upper pair spring 

 from the visceral dome. Nine apertures of inner organs are also 

 found in this cavity ; a single median anal aperture, and four paired 

 apertures, viz. one pair of genital, two pairs of nephridial, and one 

 pair of viscero - pericardial apertures. The position of these is 

 depicted in Figs. 78 and 79, p. SL'. 



The Dibranehia. 



With one exception, viz. the female .//y"/""//", which has an 

 external unchambered shell, the Dibranehia either have an internal 

 shell lying on the anterior side of the visceral dome, covered by an 

 integumental fold, or no shell at all. The visceral dome is sometimes 

 compact and pouch-like (in reptant animals, Fig. 37), sometimes, in 

 the good swimmers, much elongated dorso-ventrally, produced dorsally 

 to a point, and flattened antero-posteriorly (Fig. 34). In the latter 

 case, the body is further generally encircled by a fin-like integumental 

 fold, which marks the limit between the anterior and posterior sides of 

 the visceral dome. 



The "head" is usually distinct from the visceral dome, and carries 

 to the right and left the well -developed eyes. The mouth is sur- 

 rounded by eight or ten arms for seizing prey; these are provided 

 with suckers on their lower adoral sides. 



The mantle fold covers nearly the whole posterior surface of the 

 visceral dome, and thus encloses a very deep and spacious cavity. 

 Laterally and anteriorly to the visceral dome, the mantle fold is 

 continued as a narrow border which, immediately above the "head, ' 

 covers a shallow groove or furrow. 



The two lateral lobes which form the siphon of the Tetrabranchia 

 have in the Dibranchia grown together at their free edges, and form 

 a tube open at each end. There are only two gills in the mantle 

 cavity, one right, and one left. Near the upper siphonal aperture in 

 the mantle cavity lie the anus, and the genital and nephridial apertures 

 as well as that of the ink-bag. Details as to the arrangement and 

 number of these apertures will be given further on. 



