386 MOLLUSCA. 



The foregoing description applies to but part of the Cephalop- 

 oda. The highly different Naufcilidse have a large number of 

 lobe-like processes on the head, these without suckers. The eyes 

 are deep pits, opening to the exterior by a small aperture, the base 

 of the pit being occupied by the retina, while lens, vitreous body, 

 iris, and cornea are lacking (fig. 384). It is to be noticed that the 

 other cephalopod eyes pass through a Nautilus stage. 



In the trunk anterior and posterior sides are distinguishable, 

 the two passing into each other on the sides. The anterior side 

 (which corresponds only in part to the ventral side of other mol- 

 luscs) is wholly covered by the mantle, a strong muscular fold, 

 which takes its origin from the periphery of the body, often 

 encroaching upon the back and always terminating with free mar- 

 gins at the head. On opening the mantle by a ventral incision (fig. 

 385) the two ctenidia (four in Nautilus) are seen on either side. 



FIG. 385. Sepia officiiialis, the mantle and left nephridial sac opened to show the 

 vena cava leading to the branchial heart. , anus; 6, d, lock of siphon and mantle ; 

 (/, genital opening; K, head; fc, ctenidium ; n, nephridial sac ; n', nephridial open- 

 ing ; up, nephrostome ; t, ink sac ; !fr, siphon. 



Between them in the middle line is the anus, and right and left 

 of this and a little behind are the nephridial openings (four in Nau- 

 tilus, which also has osphradia). More laterally are the sexual 

 openings, of which one (usually the right) is commonly absent. 



