190 COMPARATIVE ANATOMY 



supporting or isolating cells can be recognised in addition to the 

 sensory eells proper ; both kinds, however, being ectodermic. The 

 mesoderm may also take part in the formation of the sensory 

 organs, giving rise to protective coverings and canals as well as to 

 contractile and nutritive elements (muscles, blood- and lymph- 

 chaanels). 



In the sensory organs of the integument of Fishes as well as in 

 all the higher sensory organs the medium surrounding the end-organ 

 is always moist. In both cases, we meet with rod-, club-, or jjeur- 

 shapcd sensory eells, but in the former the nerves coming from them 

 do not pass through nerve-cells, as they do in the organs of 

 higher sense. This indicates a lower stage of developaient, there 

 being no differentiation into sensory cell and nerve cell. 



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

 aquatic life and come on land (Amphibia) the externallayers of the 

 epidermis 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 the rod-shaped end-cell no 

 longer occurs, and two kinds of nerve-endings are seen in the skin 

 — terminal cells, and fine intercellular nerve-networks known as 

 free nerve-endings. 



Sense-Oegans of the Integument. 



a. Nerve-eminenees. 



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

 be recognised in the epidermis, especially in the anterior part 

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

 process and proximally is in contact with a nerve. The cells 

 are distributed irregularly, but in the neighbourhood of the mouth 

 and cirri they tend to form groups. 



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 to be looked upon as the sensory cells proper. The others 

 function only as a supporting and slime-secreting mass (Figs 150 

 and 151). 



In Dipnoi, aquatic Amphibia and all amphibian larvse these 

 organs retain throughout life their peripheral free position, on 



