630 PISCES. 



(1690.) No sternum, properly so called, exists in Fishes ; but the ex- 

 tremities of the ribs are sometimes connected with ossified plates belong- 

 ing to the tegumentary system, which cover the abdomen, and which by 

 some authors have been regarded as a sternal apparatus. 



(1691.) We have now to request the attention of the reader to cer- 

 tain supplementary organs which are peculiar to the class before us. 

 These consist in sundry appendages to both the superior and inferior 

 spinous processes of the vertebrae, which are generally prolonged into 

 fins situated along the mesial line of the body. These single fins, which 

 must by no means be confounded with the pairs of fins that represent 

 the arms and legs, are very variable in their position, and in many cases 

 are altogether wanting. When fully developed, one of them is situated 

 along the mesial line of the back, and in the Perch (fig. 311) this dorsal 

 fin is separated into two distinct portions (75) ; another, denominated 

 the caudal fin, forms the tail ; and a third, likewise situated in the 

 median line, at a short distance behind the anal orifice, is called the anal 

 fin from that circumstance. 



(1692.) These fins present two sets of bones the interspinous bones, 

 which form the basis to which they are affixed, and the fin-rays. 



(1693.) The interspinous bones (fig. 311, 74) form a series of strong 

 dagger-like bones, deeply implanted in the flesh along the mesial line 

 of the body, between the two great masses of lateral muscles : their 

 points generally penetrate to a little distance between the spinous pro- 

 cesses of the vertebrae, to which they are connected by a ligamentous 

 attachment ; whilst to their opposite extremity, which may be compared 

 to the hilt of the dagger, the corresponding fin-rays are affixed by a 

 beautiful articulation. There is generally only one interspinous bone 

 affixed to a vertebral spinous process, but in the Flat-fishes (Pleuro- 

 nectidce) there are two ; and moreover, in that remarkable family, the 

 inferior spinous process of the first caudal vertebra (which, -as we have 

 already seen, is of enormous size) frequently has not fewer than six or 

 seven interspinous bones appended to its extremity. 



(1694.) Each interspinous bone consists of two pieces, united by a 

 suture, one portion representing the blade, the other the handle of the 

 dagger, to which we have compared it. 



(1695.) The fin-rays of Fishes are of two kinds, being either solid, 

 and apparently composed of one strong piece, like those which support 

 the anterior half of the dorsal fin of the Perch (75), in which case they 

 are called spinous rays, or else they are composed of several slender 

 stems derived from one common root, every one of which is made up of 

 numerous pieces : these, which bear the name of soft rays, are found in 

 the posterior portions both of the dorsal and anal fin of the Perch, and 

 are invariably met with in the tail of all fishes possessed of a caudal fin. 

 This difference in the structure of the fin-rays, trivial as it might appear, 

 is a circumstance to which much importance is attached by ichthyologists, 



