18 COMPARATIVE ANATOMY. 



as well as similar elements connected with complicated pieces of apparatus 

 will be treated of later in connection with the sense-organs. 



The meaning of the "granular cells" present in the integument of 

 Petromyzon, as well as of the so-called "club-" and "goblet-cells" in the 

 many-layered epidermis of osseous Fishes, is as yet by no means clear ; it 

 is, however, not improbable that the latter of these have, to do with the pre- 

 paration of a secretion which protects the outer skin from the action of 

 the water. 



The scales of Fishes lie in connective-tissue pouches of the 

 derma, and are formed as ossifications of the latter. In Teleostei 

 they are covered by the epidermis throughout life ; in Ganoids and 

 Elasmobranchs this is only the case in the larva. (For further 

 details compare pp. 31 and 32.) 



Pigment-cells (cp. p. 16), which are under the influence of 

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

 present sometimes in both layers of the integument, sometimes in 

 the epidermis only. Muscles and glands, such as are found 

 in the skin of other Vertebrates, are not usually present 

 in Fishes. 1 



Phosphorescent organs are present in the integument of some Fishes. 



Amphibia. The structure of the integument in larval Am- 

 phibians somewhat resembles that of Fishes, while in adults it 

 more nearly approaches that seen in Reptiles. 



The epidermis of those larvas which live in the water consists 

 of two sharply differentiated layers. The outer layer is made up 

 of flat cells with a striated border (Fig. 11 A, CS) on their free edge, 

 like that already described in Fishes : the inner layer is composed 

 of more cylindrical or cubical cells (V). The former corresponds to 

 the stratum corneum, the latter to the stratum Malpighii. 



Later, with advancing development, the layers of the epidermis 

 become more numerous, and involutions towards the derma take 

 place in all parts, giving rise to a great number of globular and 

 tube-shaped glands, which are particularly abundant in certain 

 regions more especially in the head and flanks. 



Their secretion serves to keep the skin moist, but, as experi- 

 ments have shown, it also forms an important weapon of defence, 

 on account of its poisonous properties. 2 



1 There are, however, several exceptions to this rule. In male Elasmobranchs 

 there is a large glandula pterygopodii (gland of the clasper) at the base of each 

 pelvic fin. It arises as a tube-like invagination of the skin, and is in relation with 

 the copulatory organs (cp. the chapter on these organs). In the Weever (Trachimis) 

 there is a series of poison-glands lying on either side of the bases of the spines of the 

 dorsal fin ; they are situated at the bottom of integumentary sacs, and their ducts 

 open close to the bases of the spines. In Thalassophryne the operculum is pro- 

 vided with a hollow spine, at the base of which a poison-sac is situated, and in 

 Synanceia there are also a series of "poison-bags" at the bases of the grooved 

 dorsal spines (Giinther). Poison organs appear to be present in certain other Fishes 

 (many Siluroids, Aetobatis) but the existence of actual glands is not certainly known. 



The poison has no effect on other individuals of the same species ; but it acts 

 very powerfully on closely allied forms, as well as on the higher animals. 



