XIV SENSE-ORGANS 383 
even, as in Gadus, 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.* Their somatic 
sensory nerves” 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. Buil. 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. 
