62 



MOLLUSCA. 



The latter sends a large number of nerves to the mantle, the visceral 

 and the gills. The large ganglion stellatum, which is found on each \ 

 side in the mantle, a ganglion of the vena cava, two branchial 

 ganglia, and the ganglion splanchnicum are ah 1 developed on the / 



course of these nerves 

 from the viscera] 

 ganglia. 



Of the sense organs 

 the large eyes, placed 

 on the sides of the 

 head, are the most 

 conspicuous. Each eye-' 

 bulb is placed in a 

 special orbit, which is 

 partly formed by an 

 excavation in the ce- 

 phalic cartilage. It 

 is enclosed in a strong 

 capsule which is con- 

 tinued over the front 

 of the eye as a thin 

 and transparent mem- 

 brane, the cornea. The 

 cornea may, however, 

 be entirely absent 

 (Nautilus], or in other- 

 cases be pierced beneath 

 an eyelid-like cutaneous 

 fold by a small hole 

 (Oigopsidce), through 



FIG. 535. Viscera of Octopus vulgaris after removal of the which the water en- 

 posterior mantle wall and liver (after M. Edwards). m, , ,-, , 

 buccalmass; Sd>, upper salivary gland ; Oe, oesophagus, 8A, tei>S the antenor P tlC 

 Jower salivary gland; Jn, crop; M, stomach; A, end of the chamber, and 

 rectum turned back ; Oc, eye ; Tr, funnel ; Br, gills ; Ov, . f 

 ovary; Od, oviduct ; N, kidney; Kv, auricle, receiving the mto a S P ace ot 

 branchial vein; V, posterior vena cava ; C, ventricle; Ao, extent round the an- 

 terior surface of the 



bulb (fig. 534). The Cephalopod eye possesses almost exactly the 

 parts as the Vertebrate eye. The internal position of the layer of 

 retinal rods in the former may be mentioned as an essential difference 

 between them. The eye of Nautilus is without the lens. 



The two auditory sacs are placed in the cephalic cartilage, and in 



rtd 



