PHYSIOLOGY OF THE INVERTEBRATA. 365 



not only has the finer structure of the retina been affected, 

 but the shape of the optic stalks has been altered. The 

 optic stalks are not only proportionally smaller than those of 

 crayfishes possessing functional eyes, but they have in these 

 two cases characteristically, different shapes. In crayfishes 

 with fully developed eyes, the stalk is terminated distally by 

 a hemispherical enlargement ; in the blind crayfishes it ends 

 as a blunt cone. In both forms of crayfishes the optic 

 ganglion and nerve were present, the latter terminating in 

 some way undiscoverable in the hypodermis of the retinal 

 region. In G. setosus this region is represented only by 

 undifferentiated hypodermis, composed of somewhat crowded 

 cells, while in G. pelliicidus it has the form of a lenticular 

 thickening of the hypodermis, in which there exists multi- 

 nuclear granulated bodies ; these are shown to be degenerated 

 clusters of cone-cells. 



The Mollusca. 



The sense of touch, according to Gegenbaur, is chiefly 

 confined to certain cutaneous cells with setiform prolonga- 

 tions, disseminated where the body is not covered with hard 

 pieces. These cells are provided with nerves, which offer here 

 and there ganglionic expansions. In the Lamellibranchiata, 

 there are frequently tentacula around the branchial and anal 

 openings of the pallium, and the orifice of the siphon. These 

 and similar devices receive nerves from those of the pallium. 

 The tentacula and the ciliated labial palps are the tactile 

 organs of the Zamellibranchiata. 



In the Gasteropoda, represented by Selix, all the parts of 

 the body (excepting the shell) are capable of feeling when 

 touched. The tentacula, the edges of the lips, and the lobes 

 of the pallium and foot, however, have the sense of touch 

 developed in a specially high degree. 



In the Cephalopoda, the sense of touch is well developed. 

 It is situated in the arms, the fringed labial membranes, and 

 in the whole of the cutaneous covering. 



