164 



COMPARATIVE ANATOMY. 



mass grouped around the former. The central cells are in connection 

 with nerve-fibres ; each of them bears at its free end a stiff cuticular 

 hair, and they are to be looked upon as the proper sensory cells 

 (Fig. 132, CZ). The others (MZ, MZ 1 } function only as a supporting 

 mass (Fig. 135, a, b, c] 



FIG. 132. TRANSVERSE SECTION OF A FREELY PROJECTING SEGMENTAL SENSE- 



ORGAN. 



The cuticular tube and the surrounding epidermic cells are removed. 

 (sensory) cells ; MZ, MZ 1 , peripheral cells. 



CZ, central 



FIG. 133. DISTRIBUTION OF THE LATERAL SENSE-ORGANS IN A SALAMANDER 



LARVA. 



In cases where these organs project freely from the epidermis 

 and this is always the case in the embryo a delicate protective 

 hyaline tube arises from the summit; into it the terminal 

 hairs of the sensory cells project, and the tube opens distally into 

 the surrounding water (Fig. 135, R). 



While in aquatic Amphibia these organs retain throughout 

 life their peripheral free position, on a level with the epidermis, 1 

 in Fishes they may in post-embryonic time become enclosed in 

 depressions or complete canals, which are formed either by the 

 epidermis only, or, as is more usually the case, by the scales, and 

 bones of the head, and which open externally. The organs are 

 thus protected, and the hyaline tube is no longer seen. 



Those sensory organs are distributed over the whole body, but as 

 a general rule they are most abundant along certain tracts, the 

 position of which is very constant. Thus in the head, their course 

 is usually similar to that shown in Fig. 134. From this point 



1 At the time when an Amphibian undergoes metamorphosis, and gives up its 

 aquatic life, these sensory organs sink downwards into the deeper layer of the skin, 

 and, as the epidermis grows together over them, they apparently become shut ol!' from 

 the exterior and reduced, and may finally disappear. According to other authors, 

 however, they persist, and remain open, being connected with the outer surface of tha 

 skin by a tube. 



