Inteoduction.- 



-FISHES.- 



-Strcctuue. 



105 



Clenoid denotes an arrangement of processes like 

 the teeth of a comb, and designates scales with a row 

 of acute points on their margins, or on tlie concentric 

 lines of tlieir discs. These teeth or serratures are 

 almost always confined to the uncovered posterior part 

 of a scale, and do not extend to the anterior part of 

 the disc which remains in the follicle, and which is 

 most commonly radiately furrowed, as in the cycloid 

 scales. There is a general association between ctenoid 

 scales and spinous processes on the bones of the head, 

 shoulder, and gill-cover, as well as with firm and pun- 

 gent or bony rays in the fins, these pieces of structure, 

 variously combined, being more or less constantly pre- 

 sent in the order of fislies named AcanthojAeres. It 

 is to be observed, however, that with a few exceptions, 

 the skeleton of the Acanthoptures is formed of what 

 Kblliker calls a " homogeneous or tubular osteoid 

 substance " which does not contain bone-corpuscles, 

 and is not, therefore, of so truly bony a nature as the 

 skeleton of the cycloid fishes. The cjxloid and ctenoid 

 scales belong to the " True fishes " of Agussiz, and are 

 not now considered by him as of equal value for the 

 purpose of classification with the ganoid and placoid 

 kinds, which serve to characterize two groups of verte- 

 brals to which he assigns the higher value of classes. 



The ganoid scale has a hard shining enamelled 

 surface, with a bony base containing radiated corpuscles, 

 or, as Kblliker expresses it, the exo-skeleton of the 

 ganoids is composed of real bone, partly associated 

 with a liomogenous osteoid substance (ganoiu) and 

 with dentinal tubes. 



Placoid scales want the enamel of the ganoids. 

 They are sometimes in form of large plates, sometimes 

 in shape of small points — forming, when prepared with 

 the skin for use in the arts, the substance called 

 "shagreen;" at other times, they have the shapes of 

 prickles, thorns, or spines. They occur in existing 

 fishes in Agassiz's fourth class of vertebrals, the 

 Selachians. None of these fishes, according to Kolliker, 

 possess real bone-cells in their hard parts, which are 

 formed merely of cartilage-cells in a matrix indurated 

 by the deposition of earthy matter. The spines of the 

 Selachians are constructed of dentine. 



All the hard or horny productions of the skin are 

 considered, when spoken of generally, to be scales; the 

 designations of spines, thorns, prickles, bristles, hairs, 

 plates, shields, scutes, &c., being merely descriptive of 

 form. In some fishes the cranial or pelvic tones, or 

 other parts of the endo-skeleton, appear on the surface; 

 and when that is the case, their discs are very often 

 modified so as to assume the appearance and function 

 of a dermal productiun, by various kinds of sculpturing, 

 or the existence of muciparous cells, &c. The cranial 

 scutes of sturgeons, though evidently of dermal origin 

 externally, have internal surfaces of smooth compact 

 substance like an endo-skeleton. 



Fins are of two kinds, ditfering from each other in 

 their relations to the endo- and exo- skeletons. One 

 kind is always developed in the mesial plane of the fish, 

 and is formed of a duplicature of the skin either solely 

 or with the addition of rays, which may be solid and 

 pungent when they are named spines ; or binate, 

 flexible and jointed, and simple or branching, when 

 Vol. II 



they are termed soft rays. In the Dermopteres the 

 consi:t;nce of the mucoid rays is often so small that 

 they can scarcely be traced. These vertical fins appear 

 in the embryo fish as a cutaneous seam edgi])g the 

 whole upper and under outlines of the body. Parts of 

 this seam are arrested in their growth, and disappear, 

 while other parts continue to increase; and thus several 

 fins are formed. Those on the dorsal aspect are named 

 dorsals, and those on the ventral one a7ials — the mar- 

 ginal seam, with one or two exceptions only, uniformly 

 vanishing before the vent, behind which opening there- 

 fore the anal or anals are always situated. The ter- 

 minal fin is named the caudal. In some fishes, as in 

 the common eel, the continuity of the fin membrane 

 remains during life along the back, round the tip of the 

 tail, and along the ventral surf;ice forwards to the vent 

 — (Plate 1, figs. 4 and 6). Tlie caudal fin may be 

 pointed, crescentio, or forked. When the tapering 

 extremity of the vertebral column is prolonged through 

 the upper lobe of the caudal, and the lower lobe is 

 developed in its greatest breadth further forward, the 

 caudal acquires a lengthened unsymmetrical form called 

 " heterocercal" — (see Plate 17 e) — and is characteristic 

 of the fossil fishes detected in the ancient strata. 



The form of caudal which prevails in existing fishes 

 is named " homocercal," the lobes being alike nearly in 

 size and sliape, and having an even, crescentic, forked, 

 or a tapering and acute termination — (see Plate 17 c). 

 This symmetry is produced by a peculiar arrangement 

 of the irregular flat bones cafled " interneurals," to 

 which the rays of the caudal are attached; and in the 

 salmon, and many other fishes which have homocercal 

 caudals, the tip of the vertebral column may be seen 

 inclining upwards among the terminal interneurals 

 without afiecting the external equality of the caudal 

 lobes. In the mackerel tribe — (Plate 10, No. 51) — the 

 end of the back-bone is more direct, and the upper and 

 under lobes of the caudal are more distinctly separated. 

 Homocercal fishes liave not been discovered in the 

 magnesian limestone, or in any formation beneath it. 

 They make their first appearance in the oolitic strata. 



The lateral fins are not, like the vertical ones, mere 

 dermal productions, but are appendages to the endo- 

 skeleton, and are strictly homologous with the limbs of 

 a quadruped. The 2'cctorals representing the fore-feet 

 are attached to the inverted bony or cartilaginous arch, 

 composed chiefly of the coracoids, and suspended in 

 osseous fishes through the scapula and suprascapula to 

 the skull ; but having a position under the foremost 

 spinal vertcbrai in sharks, and being firmly united to 

 tliese vertebraj in the skates. 



Attached to two triangidar pieces called pubic bones, 

 the vcntrals represent the hind feet of a quadruped, yet 

 are less constant in their position than the pectorals, 

 being sometimes far behind the coracoids and connected 

 with them only by intervening soft parts, wlien they 

 are named ahdomi nals ; at other times, they are situated 

 under the face, and receive the designation ofjiiyidars; 

 and when under or nearly under the pectorals, they 

 were termed by Liunaius thoracic vcntrals. Cuvier 

 changed the latter appellation to subbrachian, and 

 extended it to all that have pubic bones joined to the 

 coracoids, even though tlie fins might be somewhat 







