TYPE MOLLUSC A. 



351 



ence to the remarkable development and differentiation of 

 the eye which is found in the majority of the Cephalopods. 



A sympathetic system of nerves is well developed and con- 

 sists of one or two pairs of buccal ganglia (&) innervating the 

 large pharyugeal mass and united to the cerebral ganglia by 

 connectives and giving rise to a strong nerve which runs 

 dorsally along the oesophagus to end in a large gastric 

 ganglion from which nerves pass to the viscera. Mention 

 should also be made of two other ganglia, the ganglia stellata, 

 which belong to the central system and are situated in the 

 lateral portions of the mantle, being united with the pleuro- 

 visceral ganglia by strong nerves ; they correspond probably 

 with tlie parietal ganglia of the Gasteropods, sending branches 

 to the tissues of the mantle. 



The special sense-organs are exceedingly well developed, 

 and especially is this the case with the eyes. In Nautilus, 

 however, the eye (Fig. 156, A] stands on a much lower grade 



FIG. 156. A, Eye of Nautilus (modified from HENSEN); and B, of Loligo. 

 c =. cartilage. I = lens. 



co = cornea, n == nerve-layer. 



g = layer of ganglion-cells. op = optic nerve and retinal ganglion. 



ir = iris. pg = pigment-layer. 



r = layer of rods. 



of organization than that of the other Cephalopods, con- 

 sisting of a cup lined by a retina composed of several 

 layers and richly supplied with nerves. The outermost 

 layer consists of rodlike bodies (r) below which is a 

 layer of pigment (pg}, below which again lies a layer of 

 ganglion-cells (g). No refractive structures are, however, 

 present, the cavity of the cup communicating freely with the 



