1.^6 On the Pelvic Fins of Onus, mustcla. 



of the following layers (fig. 9) : — Externally is the cuticle, 

 which is composed of small compressed cells. Next is the 

 Malpighian layer, which is composed of long columnar cells 

 having a peculiar beaded appearance, more conspicuous in 

 certain sections than in others. This epithelium is very 

 similar to that on the free rays of the pectoral arch of the 

 gurnard *, and seems to be higlily specialized for the purpose 

 of conveying tactile impressions. I3elow lies the corium, a 

 homogeneous layer without any obvious cellular structure. 

 Beneath this, again, is a layer of iine nerve-fibrils, which 

 supply the sensory organs. A transverse section of a free 

 ray (fig. 8) exhibits the same structure, except that the sense- 

 organs are more numerous and the nerve-supply more highly 

 developed than in the body of the fin. This points to the 

 explanation that the free tips of the fin are mainly used in 

 searching for food, and this is corroborated by actual observa- 

 tion of the living fish. 



The sensory organs {C]g. 10) have already been described 

 and figured by Bateson f in large numbers of other fishes as 

 well as in the rockling : — " They consist essentially of clusters 

 of long cells arranged together to form a bulb-shaped body, 

 whose apex is not covered by cuticle, but projects upon the 

 surface of the skin." In the rockling the base of the bulb is 

 in contact with the basement-membrane, and through this 

 the nerve enters. In certain other fishes the skin is chan- 

 nelled to receive the nerve. These organs occur in Onus 

 mustela on all the barbels, on the i3elvic fins, and also on the 

 pectoral fins, on which latter place Bateson does not appear to 

 have noticed them. In the young stages these organs are 

 absent, while in the adult, although they are most numerous 

 at the tips, they occur all over the body of the fin, especially 

 on the first fin-ray. This is connected with a change of 

 habitat, for while the young form is pelagic, the adult lives 

 below stones and in chinks in runlets, and seeks its food at 

 night by feeling along the bottom with its fins spread out at 

 right angles to the body as it swims along. It is possible 

 that these structures may be moie than tactile; they may be 

 used for tasting, since when the rockling touches anything in 

 the tank with its fins it seems to be able to distinguish be- 

 tween food-substances and others. For example, Bateson 

 noticed that when a rockling touched a piece of glass or stone 

 smeared with vaseline it turned and examined it, " clearly 

 remarking the peculiar feel of such substances." The rockling 



* II. C. Williamson, Scot. Fish. Board Eep. 1893. 

 t Journ. Mar. Biol. As.*oc. 1880-90, p. 225. 



