250 COMPARATIVE ANATOMY 



medium is always moist, and in both cases, rod-, club-, or pear- 

 shaped sensory cells are met with. 



In those animals which in the course of development give up an 

 aquatic life and come on land (most Amphibians), the external 

 layers of the epiderm dry up, and the integumentary sense-organs 

 pass further inwards from the surface, undergoing at the same 

 time changes of form. Thus from Reptiles onwards other kinds 

 of sense-organs are met with in the skin. 



SENSE-ORGANS OF THE INTEGUMENT. 



a. Nerve-eminences. 



In Amphioxus certain rod-shaped or pear-shaped cells can be 

 recognised in the epiderm, especially in the anterior part of the 

 animal ; each of these is provided distally with a hair-like process 

 and proximally is in connection with a nerve. The cells are 

 distributed irregularly, but in the neighbourhood of the mouth 

 and cirri they form groups. 



SZ StZ 



/ 



FIG. 183. VERTICAL SECTION THROUGH THE SKIN AND A LATERAL LINE ORGAN 

 OF THE LARVA OF Triton twniatus, 3 CM. IN LENGTH. (After F. Maurer.) 



BG, blood-vessel ; Ep, epiderm ; SZ, sensory cells ; St. Z, supporting cells. 



It is doubtful whether these structures in Amphioxus are 

 directly comparable to the integumentary sense-organs of Fishes 

 and Amphibians, but it is important to note that each of the latter 

 always arises in the first instance from a single cell which forms a 

 group by division. These organs always consist of central cells, 

 arranged in the form of a rounded and depressed pyramid, and 

 of a peripheral mass grouped around the former like a mantle. 

 The central cells are surrounded by a network of nerve-fibres ; 

 each of them bears at its free end a stiff cuticular hair, and they 

 are the sensory cells proper : the others have merely an isolating, 

 supporting and slime-secreting function (Figs. 183 and 184). 



In Dipnoi, perennibranchiate Amphibia, and amphibian Iarva3 

 these organs retain their peripheral free position, on a level with 



