70 COMPARATIVE ANATOMY. 



surface is striated or tuberculated, and recalls the appearance 

 of a mucous membrane. "With many species the skin forms 

 around the neck or back a fold, the posterior part of which 

 (mantle) is dilated into a hernial sac containing a portion of 

 the viscera. (Siebold.) (For shell, see p. 26.) 



Cephalopoda. Integument composed of a contractile 

 fibrous tissue, and a delicate non-ciliated lamellose epi- 

 dermis. Within the meshes of the skin a number of con- 

 tractile cells (cromatophori) are lodged, each surrounded by 

 a delicate elastic membrane. The pigment granules, which 

 they enclose, are always of the same color in each cell, and 

 produce the blue or violet or yellowish-brown spots, whose 

 extent and shade vary according as the cells are contracted or 

 dilated. (Siebold.) 



ARTICULATA. The spines, hairs, and scales of Crustacea, 

 Arachnida, and Insecta are portions of the true skeleton, q.v. 

 Glandular appendages seen in Annelida. 



VERTEBRATA. Integument composed of two well-defined 

 layers the inner, fibrous (derm, true skin); the outer, cellu- 

 lar (epidermis, cuticle). 



Pisces. The appendages to the skin form the exo-skele- 

 ton. This is composed of fins, and scales or plates; and may 

 be calcareous, osseous, or corneous. Fins are vertical folds 

 of integument, cither entirely dermal, as in Anguilla (eel), or 

 supported by bony rays, as in the majority of osseous fishes. 

 Fins are named from their locality the dorsal, caudal, anal, 

 ventral, and pectoral fins. The latter is not a true fin, but 

 in part a representative of the anterior extremity of higher 

 vertebrates. The caudal fin is homocercal when rays diverge 

 both above and below the position of vertebra, as in the ma- 

 jority of recent fishes ; or heterocercal when they diverge 

 from the under surface of the terminal vertebrae, as in Ad- 

 penser (sturgeon). Scales are organs analogous to horns and 

 nails of mammals. They may be inserted into skin each by 

 a species of socket or matrix, which is formed either in the 

 true skin or within its folds, and arranged in lamella, as in 

 majority of osseous fishes, without order, as in Anguilla (eel), 

 or in plates, as in Acipenser (sturgeon). Those with a rounded 



