162 COMPARATIVE ANATOMY chap. 



sinistrally twisted shell with the organisation belonging to a dextrally twisted 

 Gastropod, the operculum exactlj' corresponds with that of a dextrally twisted shell. 

 In Pcraclis, in the larvae of the Cymhuliichc and in Linuicina retroversa Flemming, 

 the operculum (the free surface of which must be viewed) is sinistrally twisted, and 

 the starting-point of the twist faces the (false) spiral, which in these falsely sinistrally 

 twisted Gastrojwds lies in the place of the original umbilicus. 



This apparent exception is thus shown to be quite in keeping with the rule above 

 established. 



XV. The Sensory Organs. 

 A. Integumental Sensory Organs. 



In the integument of the Mollusca there are epithelial sensory cells 

 (Flemming's cells), which vary in number and arrangement, and 

 may be scattered over large areas. Two kinds of these cells may be 

 distinguished according to their form. One kind, which is found only 

 in Lamellibranchs, consists of large epithelial cells with large terminal 

 plates which form part of the body surface and carry tufts of pro- 

 jecting sensory hairs (" paint-brush cells," Pinsel-Zellen). The second 

 kind of cells are found in all classes of Mollusca. They are long, 

 filiform, or spindle-shaped, swelling at one point where the nucleus 

 lies. They sometimes carry a tuft of sensory hairs, sometimes none. 

 Each kind of cell is continued at its base into a nerve fibre, which 

 runs into the nervous system. A distinct specific function can hardly 

 be attributed to these epithelial cells. They may respond to very 

 various stimuli, chiefly mechanical and chemical, and thus may act in 

 an indefinite way as tactile, olfactory, and gustatory cells. 



They may become more specialised in function, when crowded 

 together in certain areas of the body, and may then represent special 

 sensory organs. Between the individual cells composing such a 

 sensory organ, however, other epithelial cells (glandular, ciliated, 

 and supporting cells) are always found. 



1. Tactile Organs. 



The tactile function of the integumental sensory cells is likely to 

 assert itself at exposed parts of the body surface, such as the ten- 

 tacles, epipodial processes, siphons, at the edge of the mantle in the 

 Lamellibranchia, and at the edge of the foot, etc. We cannot, how- 

 ever, assume that even in these places the sensory cells are sensitive 

 onli/ to mechanical stimuli. 



2. Olfactory Organs. 



(a) The Osphradium. 



As has been proved to be the case in the Prosobranchia, sensory cells 

 occur scattered among the other epithelial cells throughout the whole 



