352 COMPARATIVE ANATOMY. 



ones, are capable of feeling when touched; the anatomical arrange- 

 ments for this sense are found in various parts of the body, through 

 which they are more or less widely distributed ; they have the form 

 of fine setiform prolongations from cells, which can be seen to be 

 connected with nerves. These structures are most well marked on 

 those parts of the body which function especially as tactile organs ; 

 they are generally supplied with nerves of some size, and form the 

 tentacles. 



They are very common on the edge of the mantle in the 

 Lamellibranchiata, where they are either found all over it, in 

 which case they are arranged in several rows (as in Mactra, Lima, 

 Pecten, etc.), or sometimes they are confined to certain regions ; 

 they are not unfrequently found on the siphons, and in either case 

 they serve to watch over the particles that get into the mantle-cavity 

 with the water. They are highly contractile, and are supplied with 

 filaments from the marginal nerves of the mantle. 



The processes found on the epipodium, and on the edge of the 

 mantle in many Gastropoda, as well as the dorsal cirri of the Nudi- 

 branchiata, may function as organs of this kind. 



It is doubtful whether the pair of lobes at the sides of mouth of 

 the Lamellibranchiata are organs with this function, but on the other 

 hand we find a very large number of tactile organs of this kind on 

 the cephalic tentacles which are so commonly present in the Gastro- 

 poda. The tracts, which carry the nervous end-organs, are very 

 often specially differentiated in these animals. 



Although there is not much difficulty in the view that the above- 

 mentioned organs function in the perception of tactile impressions, 

 it is almost impossible to say what is the physiological duty of a 

 number of other organs, which are clearly sensory, and are connected 

 with the integument. These enlargements are generally formed by 

 ciliated regions to which a nerve passes, and at which it often forms 

 enlargements. It is doubtful what part of the surrounding medium 

 acts on these organs, and we have to make a somewhat far-fetched 

 analogy to be able to regard them as olfactory organs. 



In the Gastropoda they are found near the respiratory organs, 

 and in the Heteropoda I found them very widely distributed in this 

 region. The same I found to be the case in the Pteropoda. In the 

 Gymnosomatous forms of this division a ciliated organ of this kind 

 is placed superficially, and close to the gills; in Pneumodermon it is 

 wheel-shaped. In the shelled forms it is a transverse ridge, which 

 is placed in that region of the mantle-cavity by which the water 

 passes to the branchiae. 



In the Opisthobranchiata the hinder pair of tentacles (rhinophor) 

 appear to have the function of an olfactory organ ; in correlation 

 with this function they vary greatly in character, and the surface of 

 these tentacles may be seen to be increased by the formation of 

 ridges and various other arrangements. They seem to be in all cases 

 ciliated.. If we reflect on the fact that respiration is largely carried 

 on by organs which arise from the back of the animal, we shall see 



