SENSE-ORGANS 383 



even, as in Gaclus, to all the fins, especially to the numerous 

 "end -buds" which are present on those organs. In many 

 Fishes a variable number of the anterior spinal nerves (spino- 

 occipital'] perforate the occipital region of the skull. They 

 probably represent the ventral roots only of the ordinary spinal 

 nerves of this region. 



SENSE-ORGANS 



The Cutaneous Sense-Organs, These organs, the most re- 

 markable and certainly the most characteristic of the sense-organs 

 of Cyclostomes and Fishes, are bud-like groups of epidermic cells in 

 relation with the ends of sensory nerve fibres. Each consists of a 

 central core of sensory cells, provided with terminal cuticular sensory 

 hairs, and surrounded by a zone of supporting and mucus-secret- 

 ing cells which leave the hairs exposed at the apex of the bud. 

 Two kinds of these organs can be distinguished, which differ in 

 their innervation and in their position in the skin. Of the two, 

 the so-called end-buds are the more primitive. They occupy a 

 superficial position in the epidermis, and their sense-cells are as 

 long as the supporting cells. They are present in Cyclostomes 

 and Elasmobranchs, and especially in Teleosts, where they are 

 irregularly distributed over the surface of the body, on the fins, 

 lips, and barbels, and also in the epithelium of the mouth and 

 pharynx. In the Dipnoi they are limited to the oral cavity, and 

 in the higher Craniates they become taste-buds. 1 Their somatic 

 sensory nerves 2 are derived from the vii., ix., and x. cranial nerves, 

 and the lateralis accessorius. In the second type, usually 

 called " nerve-eminences" the sensory cells are shorter than the 

 supporting cells, and they are always innervated by the lateralis 

 system. When first developed in the embryo they are quite 

 superficial, like end -buds, but later the epidermis in which 

 they lie sinks inwards so as to line a series of pits, closed sacs, 

 tubes, open grooves, or closed canals. Pit-organs, so abundant on 

 the head and trunk of Teleosts (Fig. 220), are simple epidermic 

 pits with insunken nerve-eminences, disposed in groups or in 



1 For a discussion of the relations of "end -buds" to the sense of taste in 

 Fishes, see Bateson, Journ. Marine Biol. Ass. i. (N.S.) 1890, p. 225 ; and Herrick, 

 U.S. Fish Commiss. Bull. 1902, p. 237. In the latter paper a bibliography of the 

 subject is given. 



2 These fibres are included in the visceral sensory or "communis" system by 

 Herrick. 



