320 Mollusca. 



and is further provided with a lens; the inner half of which is 

 secreted by the epithelium of the optic vesicle ; the outer half by the 

 epidermis. Moreover, at the periphery of the eye, a large fold like 

 an eyelid is present, forming a cavity round the eye ; in some 

 Decapoda {(Ugopsidce, Fig. 265 B) this cavity is widely open ; in the 

 rest {Myopsidce) and in the Octopoda (Fig. 265 C) the fold extends 

 completely round the eye, and the aperture leading into the cavity 

 is very small; where the fold lies above the lens it is transparent, 

 and is termed the cornea. In the cavity thus formed there 

 is a second small pigmented fold, which displays a certain resem- 

 blance to the iris of the Vertebrata, and is also designated by 

 that name. A depression of the skin situated laterally on the head 

 behind the eyes and supplied by a nerve from the brain, is regarded 

 as an olfactory organ. 



The mouth is surrounded by a projecting fold of skin, the lip, 

 within which are two powerful horny jaws, an upper and a lower; 

 the former bites within the edge of the latter, and the two together 

 are very similar to a parrot's beak inverted. In the mouth, which is 

 furnished with muscular walls, there is a radula like that of the 

 Gastropoda. The anus lies far forward on the ventral side of the 

 body in the mantle-chamber, in the median line. A stomach, a large 

 liver, and usually salivary glands are present. The heart consists 

 of a ventricle and as many auricles as there are gills, that is. 



Fig. 266. Diagram of the heart, etc. of a Cephalopod. h ventricle, / auricle, u, « 

 arteries, vh branchial heart, of vein to the giU, vf vein from the gill, g gill. — Orig. 



four in Nautilus, two in other Cephalopods. In the Dibranchiata 

 the large veins which carry the blood to the gills are known as 

 branchial hearts; they are enlarged at the entrance of the gills 

 and contractile. The kidneys, two pairs in Nautilus, one pair in 

 the Dibranchiata, are saccular organs openiug into the palHal chamber 

 by paired apertures. In some of the Dibranchiata the two are 

 partially fused, but each has its own opening. The kidneys exhibit 

 racemose evaginations of the large adjoining veins which have 

 pushed the closely attached wall of the kidney into its cavity ; these 

 evaginations appear to hang freely into the cavity. 



