428 TEGUMENTARY ORGANS 



cally, if not solely, developed in the class of Fishes — the so-called 

 mucous canals and follicles. It has long been noticed, in fact, that in 

 osseous fishes one series of the scales along the sides of the body 

 differ in their structure from the rest, giving rise to what is called the 

 lateral line ; and that a canal runs beneath these scales from the tail 

 to the head on each side ; that then becoming connected with its fellow 

 by a transverse branch over the occiput, each canal passes forward on 

 the sides of the head, dividing into two principal branches, one of 

 which following the course of the suborbital bones terminates at the 

 end of the snout, while the other passes down on to the lower jaw. 

 Similar organs, but having a more complicated arrangement, are 

 known to exist in the cartilaginous fishes ; but it is commonly 

 supposed that these canals and follicles secrete the mucus with which 

 the skins of fishes are lubricated. However, in a very beautiful series 

 of researches, Leydig has shown that the mucus is furnished by the 

 cellular ecderon, and that the so-called mucous canals and follicles 

 are sensory organs. The limits of this article will not permit me to^ 

 enter into any of the details of structure of these organs, but they 

 may all be described generally as sacs or canals lined by a cellular 

 investment, like that of the skin upon which they open, and filled 

 with a more or less gelatinous substance. If the organ be a sac, a 

 single protuberant knob, if a canal, a series of them project into the 

 cavity. Each knob is covered by a coat consisting of tiers of much- 

 elongated cylindrical cells. Its substance consists of more or less 

 gelatinous connective tissue, and it receives a nerve (a branch of the 

 fifth or of the vagus), whose fibres divide and become lost in its 

 tissue. In the osseous fishes this nerve usually perforates the peculiarly 

 modified scale of the lateral line, which supports and encloses the 

 canal at these points. In the cartilaginous fishes the canals have 

 sometimes special fibro-cartilaginous coats ; or if sacculi, a number of 

 them may be contained in a common cartilaginous investment, as in 

 the Chimjera. Leydig insists with great justice on the identity of the 

 structure of these organs with that of the semicircular canals of 

 the ear. 



The connection of these sacs and canals with the corpuscula tactus 

 and Pacinian bodies appears to me to be clear ; for the knob which 

 projects into the cavity of the mucous canal is homologous with the 

 central " nucleus " of the Savian body, and this with the solid axis of 

 the Pacinian body, and with the corpusculum tactus, so that the 

 " tactile " sac of the Chimaera, e.g., may be said to be-a tactile corpuscle 

 which is connected with the surface of the integument. 



No organ at all resembling these has certainly been met with. 



