NATURAL HISTORY OF VERTEBRATES. 



ep. 



Fig. 6. — Diagrammatic section of the skin of a fish ; 

 c, corium ; c^, clavate ceils ; eb, end-bud, or goblet organ ; 

 /, free neuromast ; m, mucous cell ; n, nerve ; p^ pigment 

 cell ; s, saccular neuromast ; i-c, mucous canal ; «, vein ; ep^ 

 epidermis. 



the above, and are often segmentally disposed. They are either free on the surface, 

 as in the blind fishes, or retracted more or less within sacs, as in the sturgeon and cat- 

 fish, or finally withdrawn from the epidermis entirely, and lying within canals in the 

 corium which course through the head and along tlie lateral line of the body. Such 

 neuromastic canals are present in fishes, with few exceptions; they are generally 

 strengthened by bone round the place where the neuro-epithelial patch is situated, and, 



in fact, important scales and bones are 

 developed with such organs as their 

 centre of formation. Fig. 6 represents 

 the three chief types of neuromasts 

 met with in fishes. No such structures 

 occur in the higher Vertebrata, unless 

 the auditory organ is to be conceived as 

 a very highly specialized grouj) of pro- 

 tected neuromasts. Apart from the 

 similarity in the structures of the neuro- 

 epithelium, other theoretical considera- 

 tions favor this view. It has been in- 

 dicated that the skin of air-breathing 

 vertebrates differs in many respects 

 from that of aquatic forms. Very early 

 that layer of the epidermis which cor- 

 responds to the most superficial stratum in the aquatic forms is thrust off, and 

 underneath it one or more layers of cells dry up, flatten, and undergo a chemical 

 change, which process results in the differentiation of two strata of epidermal cells, 

 the superficial horny and deep mucous strata. The flattened scale-like cells of the former 

 are constantly being rubbed off (or exuviated in greater masses, as in the snake's slough) 

 and replaced by the cells of the lower layer, which retain the power of dividing. It is 

 obvious that the formation of this horny layer is in direct response to the change of 

 medium in which the animal lives. 



Associated also therewith is the disappearance of the delicate neuro-epithelial struc- 

 tures discussed above. Although the nerves may still be traced into the cells of the 

 mucous layer, yet specialized end-organs are now found only in the corium, where they 

 occur in very various grades of evolution. In all, however, the sheath of the entering 

 nerve-fibre is very much thickened, while the individual cells of the altered sheath are 

 disposed either longitudinally or transversely in the club-shaped or globular ' tactile 

 corpuscles.' 



The lubrication of the surface effected by special cells in the lower forms is, in 

 the Amphibia, brought about by the secretion of cutaneous glands. These are formed 

 from the mucous layer of the epidermis, which may project inwards to some extent into 

 the corium, and even receive a contractile sheath from it which regulates the discharge 

 of the glandular secretion. They may occasionally take qn special functions, as in the 

 poison-glands of certain toads, etc. Such cutaneous glands are rare in the reptiles and 

 birds, but they reappear in several distinct forms in mammals, where they associate 

 themselves with very various functions. 



Thus the sweat glands of mammals are important organs of excretion, assisting 

 the kidneys in removing waste material from the body. Their deep position in the 

 corium, although formed from the mucous layer of the epidermis, is characteristic. 



