TEGUMENTARY SYSTEM. 185 



and they are here perforated by a canal, or frequently by a short 

 tube, through either of which the mucous ducts, presently to be de- 

 scribed, open externally upon the integument. 



The bony scales of many Fishes, e. g., Lepidosteus, Polypterus, 

 Trigla, differ from the ordinary scales of the Osseous Fishes, for in 

 them we find distinct osseous corpuscules. True tegumentary bones 

 occur in the Sturgeons, many Siluri, and in the genera Polypterus 

 and Lepidosteus, and form large bony plates, which are frequently 

 invested by enamel. In the Trunk-fish (Ostracion) these form 

 tolerably regular six-sided plates, which are so accurately fitted to 

 each other, as to form a very compact and hard coat of mail. In 

 the Spinous Globe-fishes (Diodon, Tetrodon), the scales project from 

 the surface of the body into long and pointed spines. Small acumi- 

 nate elevations are seated upon the integument in the Rays and 

 Sharks, and in the intervals between them, in the Rays, several lar- 

 ger ones, situated upon a broader basis, are prolonged externally into 

 a transparent spine, and exhibit internally, like the teeth, a medullary 

 pulp, to which vessels are distributed. 



The integument is traversed by the peculiar narrow Mucous Ca- 

 nals, which give off short, transversely-directed branches, and termi- 

 nate externally by free open mouths, in different situations, but espe- 

 cially upon the lateral line, on the head and snout, e. g., in Fishes 

 of the Eel kind. In the Gadus merlucius, in which these mucous 

 canals have been most carefully examined, there is one which runs 

 like a vessel along the whole length of the body, bifurcates behind 

 the eye, and gives off a pair of branches to the snout, while at inter- 

 vals branches arise from it that open upon the integument ; a small 

 twig also passes over the praeoperculum to the lower jaw. In the 

 Rays and Sharks particularly strong and tortuous canals are found 

 imbedded in the integument of the head. In Torpedo two rows of 

 openings lie upon either side of the back, and open into two cor- 

 responding longitudinal canals. Special layers of glands are situ- 

 ated beneath the lateral line, and are, in the Carp, Tunny, and 

 several other Fishes, much developed. They appear to secrete the 

 mucus, which passes out through the canals traversing the scales 

 of the lateral line. In rare cases these mucous canals are wanting, 

 as in some, but not all, the genera of the order Cyclostomi. The 

 mucous canals of the head are frequently covered by hard scales, 

 which serve, to protect them ; this is the case throughout the whole 

 course of the mucous canals in Polypterus Bichir. In other in- 



