410 THE ANIMAL KINGDOM. 



Epidermic sensory organs are largely developed. The azygos fins are 

 supported by fin-rays, as also are the borders of the paired limbs ivhich have 

 the form of fins. There is no Eustachian tube in the proper sense, no 

 tympanic membrane or cavity, nor stapes in relation with the ear. The 

 alimentary canal terminates very generally in a cloaca, or else the anus lies 

 in front of the genito-urinary apertures. There is no vena cava inferior. 

 An allantoid outgrowth of the digestive tract is invariably absent. 



The epidermis consists of polygonal cells in Elasmobranchii. It 

 never developes a stratum corneum, but in some Teleostei and the Dipnoi 

 the superficial layer of cells has a striated cuticular border. Flask-shaped 

 mucous cells with the narrow end internal are very generally present, and 

 in large numbers in some Teleostei and the Dipnoi. They occur in 

 Acipenser (Ganoidei], but not in Elasmobranchii. They appear to rise 

 to the surface and there burst. Branched cells, both pigmented and non- 

 pigmented, are found both in the epidermis and the cutis. They are 

 connected with nerve-filaments. Some Fish, e. g. Turbot and Trout, have the 

 power of altering their colour in relation with the nature of the bottom 

 on which they are living. The epidermis at the breeding season often 

 assumes a silvery appearance, and in some instances a brilliant coloration, 

 e.g. Stickleback (G aster osteus). The epidermic sensory structures are 

 nerve-eminences and end-buds. The former are rarely absent altogether 

 (Balistes) ; they may be generally distributed over the body, e. g. Mugil, 

 or more or less limited to certain lines. These are typically, on the head 

 a supra- and infra-orbital, and a mandibular line on each side, with a 

 connecting supra-occipital line ; on the body a lateral line, which follows the 

 plane of division between the dorso- and ventro-lateral muscles, and more 

 rarely a second line placed more on the dorsal aspect, and a third nearer 

 the ventral surface. Scattered eminences may occur at the same time, 

 e. g. Pike (Esox). They either project freely above the surface, or are sunk 

 in grooves of the scales or in integumentary pits when scales are absent, 

 e. g. Silurus glanis ; or finally are lodged in canals which open externally 

 from spot to spot. Echinorhinus spinosus and a few others among Sharks 

 and the Holocephali have exceptionally an open groove in the region of 

 the trunk. Canals are generally developed on the head, where they may 

 be much branched as in Elasmobranchii ; on the body also, but either only 

 anteriorly, or posteriorly (some Teleostei], or throughout its whole length. 

 The canals of the body are much branched in the Rays. The whole canal 

 system and integumentary pits are filled with mucus secreted by certain 

 of the lining cells. This mucus is gelatinous and firm in Elasmobranchii, 

 softer in other Fish. Elasmobranchii have peculiar structures known as 

 ampullae on the head, and Ganoidei nerve-sacs in the same region. The 

 latter are minute depressions with sense-cells at the bottom, beneath which 

 lies a space filled by jelly. The former are tubular, and the base of the 



