The fins usually present are of two kinds the paired fins and the vertical fins. The paired fins are represented on either side of the body, the anterior or uppermost pair, the pectoral fins, representing in the fish the arms or forelegs of the vertical fins, supporting bones are devel oped. These bones, called interspinal bones (niterneural along the back, interhminal be low), stand in some definite relation to the bones of the vertebral column, to which, how ever, they are not directly joined.
The pectoral fin is attached, by means of certain intermediate bones, to the shoulder girdle. The ventral fin is similarly joined to a pelvis. The pelvis in all the lower fishes is detached from the rest of the skeleton and placed beneath the body cavity. In the more specialized modern forms the pelvis is moved forward, usually connected with the shoulder girdle, but sometimes attached at the throat well in front of the latter. In accordance with the insertion of the pelvis, the ventral fins are said to be abdominal (free from the shoulder girdle, as in the whitefish), thoracic (con nected with the shoulder-girdle), or jugular (in front of the shoulder-girdle).
the higher vertebrates, the lower or posterior pair (ventral or pelvic fins) representing the hinder legs. The verticle fins are on the median line of the body, the dorsal on the back, the caudal on the tail and the anal on the lower side behind the body cavity. Each of these fins is subject to great variation.
The vertical fins, with their appendages, arise from the modification and specialization of a dorsal median fold of skin. This is found in embryos and in the lancelet and other forms of low organization. It is generally taught that the paired fins, with the bones which support them, have been derived from a lateral fold of skin, similarly modified, and still more specialized. As to this, the evidence is much less complete than in the case of the vertical fins. But there is some evidence which would show that the paired fins arose from modifica tion of gill structures.
At the base of each ray or group of rays in Muscular System of the Fish.— The move ment of the fins is accomplished through the agency of muscles. These organs lie band-like along the sides of the body forming the flesh of the fish. They are little specialized and not
as clearly differentiated as in the higher verte brates. In the typical fishes there are several distinct systems of muscles, controlling the jaws, the gills, the eyes and the different fins. The largest of these is the great lateral muscle, composed of flake-like segments, these cor responding in general in number to the num ber of vertebra:. In general the muscles are white in color and tender in substance. In a few fishes muscular tissues are modified into electric organs.
The skeleton of the fish is cartilaginous in the more primitive forms, bony in the more specialized types. In all cases the bones con tain less of mineral matter than do the bones of the higher vertebrates.
Skeleton of the Fish.- In the process of development the vertebral column is built up about a soft cartilaginous cord extending lengthwise of the body, called the notochord. This exists in all young fishes and is more or less completely persistent through life in the more primitive types, as the lamprey or the sturgeon. This notochord is not the backbone, but it indicates where the backbone is to be. In the higher forms the vertebra are developed around it. strung spool-fashion, as it were, until finally in most recent fishes the original chord is entirely obliterated.
Explanation of plate of the skeleton of the striped bass (Roccus lineatus) after Edwin Chapin Starks.
I. Vomer. 44. Suspensory pharyngeal.
2. Ethrnoid. 45. Upper or Superior 3. Prefrontal. PharYnatalls.
4. Frontal. 46. Lower or Inferior 5. Sphenotic. pbaryngeals.
6. Parietal. • 47. Gill-rakers.
7. Epiotic. 48. Supraorbital.
8. SupraoccipitaL 49. Preorbital.
9. Pterotic. 50. Suborbitals.
lO. Opisthotic. Si. Nasal.
l 1. Exoccipital. 52. Supra-temporal.
l2. Basiocapital. S3. Post-temporal.
l3. Parasphenoid. 54. Supraclasncle.
14. Besisphenoid. SS. Clavicle.
lS. Proottc. 56. Postclavicle.
16. Alisphenoid. 57. Hypercoracoid.
l7. Hyomandibular. 58. fiypocort.
lg. 59. Mes000mcoid.
1.9. te. 60. Actnosts.
10. ygoid. 61. Pectoral rays.
21. Palatine. 62. Pelvic girdle.
12. Mesopterygoid. 63. Ventral spine.
M. Metapterygoid. 64. Abdominal vertebra.
t4. Preopercle. 65. Caudal vertebne.