2*0 COMPARATIVE ANATOMY 



facial, and spinal nerves ; and a number of accessory parts may be 

 present, e.g. a pigment- layer, a reflecting apparatus, a vitreous 

 body, and structures resembling a lens and an iris. These organs 

 probably serve to attract prey or to help their possessors in seek- 

 ing food in the darkness of the deep sea. 1 



In addition to very numerous goblet-cells, the epiderm of the 

 South African Dipnoan, Protopterus, gives rise to cup-shaped 

 multicellular glands, resembling those of the Amphibia. During 

 the dry season, this animal (like its South American ally, 

 Lepidosiren 2 ), buries itself in the river-bottom : its integumentary 

 glands then produce a varnish-like secretion and an enclosing 

 cocoon or capsule, by means of which it is protected during the 

 torpid period. 



Pigment-cells, which, as already mentioned, are under the 

 control of the nervous system, and are able to cause a change of 

 colour, are present sometimes in both layers of the integument, 

 sometimes in one only. Colour may also be produced by reflecting 

 bodies consisting of excretory products (guanin) and known as 

 iridocytes. 



The presence of scales (see under Exoskeleton) may affect the 

 epiderm when they project from the surface, and in some cases it 

 may disappear so that the scales become superficial (e.g. in Elas- 

 mobranchs, Ganoids, and some Teleosts). 



Amphibians. 



The epiderm of Amphibians differs markedly from that of 

 Fishes, inasmuch as nearly all the special forms of cells so 

 characteristic of the latter are wanting. Both epiderm and derm, 

 moreover, differ in the larva and in the adult. The epiderm at 

 first consists of a single layer of cells, and then of two layers, the 

 superficial one being provided with a ciliated or a striated cuticular 

 border, 3 and remaining throughout the larval period as a covering 

 layer (Fig. 14). The deeper layer, on the other hand, undergoes 

 various modifications : it becomes stratified, and replaces the super- 

 ficial cells as they are lost. Slime-secreting goblet-cells, such as are 

 characteristic of the epiderm of fishes, are typically wanting, 4 and 

 leucocytes are not abundant. Unicellular glands, known in 

 Urodeles as Leydig's cells, are, however, abundant in the larva ; 



1 For the electric organs of Malopterurus, which are said to be epidermic in 

 origin, cf. under Electric Organs. 



2 In the breeding season the posterior extremities of the male Lepidosiren are 

 provided with numerous long vascular papillae. 



3 Cilia occur abundantly in Salamander larvse over parts of the head and 

 body, and their distribution is related to that of the integumentary sense-organs ; 

 they are also found in very young Anuran larvse. 



4 In older Urodele larvie, after the epiderm has become thickened, numerous 

 goblet-like cells can be recognised and probably represent the Leydig's cells 

 described above. 



