628 PISCES. 



sweep of their expanded tail, which forms the principal agent in pro- 

 pelling them forwards, is far more essential to their habits. Thus the 

 cartilaginous spine of the feeble Lamprey is sufficient for all needful pur- 

 poses ; and even in the most perfectly ossified fishes, from the manner 

 in which the vertebrae are united to each other, the greatest possible 

 flexibility is ensured. The body of each vertebra presents two conical 

 cups, the apices of which are nearly or quite continuous ; the margin of 

 each cup-like depression is united by elastic ligament to the correspond- 

 ing margin of the contiguous vertebra, and thus between the bodies of 

 each pair of vertebrae a wide cavity is formed (fig. 310, D), which is filled 

 up with a semigelatinous substance; so that, by this beautiful con- 

 trivance, the mobility of the whole chain is abundantly provided for. 



(1687.) There are only two kinds of vertebra recognizable in the 

 skeleton of a Fish, viz. the abdominal and the caudal. The abdominal 

 vertebrae support the ribs (for in these animals the ribs do not constitute 

 a thorax, or contain any of the viscera called thoracic in the human 

 body) ; they extend from the head to the commencement of the tail, and 

 are at once recognizable by the nature of the elements which enter into 

 their composition, each vertebra being provided with a superior arch 

 (fig. 306, 6), through which passes the spinal cord, a superior spinous 

 process (c), and two transverse processes (c), to the extremities of which 

 the ribs are generally attached. The caudal vertebree are composed, as 

 we have already seen, of different elements : the transverse processes 

 either do not exist, or are very feebly developed ; 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 

 (fig. 311, 6). 



(1688.) As the vertebrae approach the tail, they become somewhat 

 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 (fig. 311, 70). 



(1689.) 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 

 imbedded among the lateral muscles of the trunk, to which they give 

 support. From each rib arises a long styliform process (73), which, in- 

 clining backwards, is likewise plunged among the muscular fasciculi ; 

 and in some fishes, such as the Herring and Carp tribes, similar append- 

 ages are derived from the bodies of the vertebrae themselves, so that the 

 bones of such fishes appear to be extraordinarily numerous. On the 



