30 MONOGRAPH OF THE FRESH WATER III. 



terrupted between the rays of the dorsal fins. The same is observed on the lower 

 part of the toil from the anus to the caudal fin. On the abdomen and belly the 

 layer is so thin that the flakes are exposed as soon as the skin is removed. 



9. THE SKIN AND THE LATERAL LINE IN COTTOIDS. 



The skin in Cottoids is generally scalcless, and, in most cases, smooth. The 

 Hemilepidoti of the marine tribe constitute the single exception, and even here 

 they exist merely in bands or patches scattered over the body. The fresh-water 

 species, however, are absolutely scalelcss. 



In several Cotti, the skin on the thoracic region is beset with very minute 

 asperities, perceptible only to the touch when the finger is passed from behind 

 forwards. 



On examining under the microscope the skin overlying the head of Cotti, we 

 observe in its thickness small irregular star-like ossifications which are more 

 developed in Acanthocotti so as to become sensible both to the touch and naked 

 eye. In Triglopsis, the skin does not, even under the microscope-, exhibit any- 

 thing in its muscular texture; whilst in Cottopsis its whole surface is prickly from 

 head to tail. There can be no doubt that those prickles are the same parts which 

 we observe on the thoracic region of Cotti, only in Cottopsis they assume their 

 maximum of development. Not having the opportunity of examining their struc- 

 ture in Cottopsis, we cannot venture any opinion as to their signification; whether 

 the homologues of true scales or a production of the skin peculiar to fishes 

 unprotected with scales. 



The lateral line in Cottoids has a very remarkable structure, which could hardly 

 have been suspected had the discovery of the genus Triglopsis not been made. Its 

 beauty and development in the latter is such as to leave no doubt that it forms a 

 regular, cartilaginous tube, with a series of quite large openings outwards (Plate II, 

 Fig. 11, 1 1) communicating through a corresponding series of pores of the skin, 

 with the surrounding medium. The removal of the skin exposes this tube, which 

 still adheres to the sides by the layer of dermic muscles ; after this layer is removed, 

 the cartilaginous tube may be detached from the tail forwards, where it is seen to 

 be united with the head. It passes through a channel of the occipital bones, sending 

 down a branch along the preopercular, into the lower jaw. Proceeding forwards it 

 branches off again in advance of the orbit, one branch running along the upper 

 edge of that cavity into the nostrils, the other following its lower edge through the 

 chain of suborbital bones into the nostrils also. 



That the lateral line in Triglopsis is intended to supply water to the s\>tein, 

 there can be no doubt ; and as little, that it answers the same end in other fishes. 

 Its structure may be diversified according to the natural groups ; this constitutes 

 it- morphology ; but its philosophic meaning i.s the same throughout the whole 

 range of the class. 



