492 PISCES FISHES. 



veloped ; but beneath the body an inferior arch is formed, and 

 from this an inferior spinous process, equalling the superior in 

 length, is prolonged in the opposite direction (Jig- 221, b). 



(526.) As the vertebrae approach the tail, they become some- 

 what modified in structure to support the caudal fin ; their spines 

 become shorter and thicker, the canals formed by their superior 

 and inferior arches smaller or nearly obliterated, and at length 

 the spines become, as it were, soldered to each other, and to the 

 interspinous bones hereafter to be noticed ; so that they form a 

 broad vertical plate, to the posterior margins of which the rays of 

 the tail-fin are articulated (Jig. 221. 70). 



(527.) The ribs of fishes are slender bones, appended either 

 to the extremities of each transverse process of the abdominal 

 vertebrae, or else to the body of the vertebra itself : every rib is 

 connected with but one vertebra, and that only at a single point. 

 They do not, as we have already said, form a thoracic cavity ; but 

 enclose the abdomen, and are embedded among the lateral muscles 

 of the trunk, to which they give support. From each rib arises a 

 long styliform process (78), which, inclining backwards, is likewise 

 plunged among the muscular fasciculi ; and in some fishes, such as 

 the Herring and Carp tribes, similar appendages are derived from the 

 bodies of the vertebrae themselves, so that the bones of such fishes 

 appear to be extraordinarily numerous. On the other hand, many 

 tribes have but the rudiments of ribs ; and in some, as for example 

 in the Skate, they are altogether wanting. 



(528.) No sternum, properly so called, exists in fishes ; but 

 the extremities of the ribs are sometimes connected with ossified 

 plates belonging to the tegumentary system, which cover the abdo- 

 men, and which by some authors have been regarded as a sternal 

 apparatus. 



(529.) We have now to request the attention of the reader to 

 certain 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 pro- 

 longed into fins situated along the mesial line of the body. These 

 azygos fins, which must be by no means 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 mesian line of 

 the back, and in the Perch (Jig- 221) this dor sal Jin is separated 

 into two distinct portions (75) : another, denominated the caudal 



