STRUCTURE Of FISHES. 



3 



In the matter of mental endowment fishes are probably but little, if at all, inferior to the majority 

 of the so-called higher animals. The angler knows well their caution, discrimination, cunning, 

 and boldness, and how often his own powers and patience are exerted in vain in entrapping a fish who 

 has grown wise as well as old in observation of the phenomena of the river in which he lives. 

 Fishes would appear to be capable of affection, since Sharks, at least, frequently swim in pairs. 

 Some genera are capable of being trained, and a few are known to be gifted with vocal organs 

 which, to judge from the analogy of higher animals, may fairly be regarded as a means for the 

 inter-communication or expression of emotions and experiences. Removed as fishes are from the 

 conditions of daily observation by living in water, fewer observations have necessarily been made 

 upon their intellectual characteristics than is the case with animals which can be more easily 

 studied. 



In their general structure or anatomy fishes are usually well distinguished from other animals. 

 Their most distinctive structures are, perhaps, the possession of gills and an air-bladder. But they 

 are no less well defined by the 

 peculiar forms assumed by the 

 limbs which we call fins, and by 

 the simplicity of the plan upon 

 which the immense muscles, 

 which form the larger part of 

 the fish's body, are arranged. 

 The variety in structure, how- 

 ever, presented by fishes is so 

 great that the lowest type 

 represented by the Lancelet 

 seems almost to pass beyond 

 the limits of the fish group, 

 standing alone in its simplicity 

 and in many details of structure 

 in which a parallel can be traced 



with yet lower animals. Other fishes also diverge so far from the typical forms as to possess 

 lungs, as may be seen in the Ceratodus of the Aiistralian rivers, and in the Mud-fish, called 

 Lepidosiren. It may be useful briefly to mention the chief characteristics of the several groups of 

 organs of this class of animals. 



In the lowest type of fish, of which the Lancelet (Amphioocus lanceolatus) is the only repre- 

 sentative, the cranium is merely a forward continuation, of the rod which represents the vertebral 

 column. This rod is named the notochord, or chorda dorsalis, and consists of a fibro-gelatinous 

 substance, which is not covered with cartilage or with bony matter. This gives a very imperfect 

 conception of the skull as usually seen in fishes ; yet a jointed cartilaginous arch extends downward 

 round the region of the mouth, and is a foreshadowing of the arch which is more perfectly 

 developed around the mouth in the Lampreys. In simplicity of skull-structure the Sharks and Rays 

 are the next step in the upward series, but thece are many points in connection with these animals 

 which lead to the belief that they are among the highest types of fish. The notochord is now 

 converted into firm granular cartilage, sheathed in bone, and divided into segments by bone de- 

 posited in its substance ; but it extends forward along the base of the skull, and develops two 

 oblong convex surfaces, which are termed the occipital condyles, by which the back of the skull 

 unites with the first vertebra. This mode of union of the skull with the vertebral column is 

 characteristic of amphibians and mammals, and since the other Vertebrates have the skull united 

 to the vertebral column by a single occipital condyle, it has sometimes been thought that we may 

 discern herein a special indication of affinity between the skulls of Sharks and those of the 

 Amphibia, which, it will be remembered, possess, when they commence their existence, many of 

 the structures of fishes. There is no distinction of bones, however, in the brain-case, but the bony 

 matter is deposited in countless little cells. Its base is flat ; the sides are contracted ; it is usually 

 flattened above, with one or more open spaces, or fontenelles, which are covered only with 



SKELETON OF THE COMMON PERCH. 



a, Pre-inaxillary Bone: 6, Maxillary Bone; c. Under Jaw; d, Palatine Arch; e. Cranium;/, Inter- 

 operculum : g </', Vertebral Column ; h, Pectoral Fin ; i, Ventral Fill ; k, I, Dorsal Fins ; m. Anal Fin ; 

 '. Candal Fin. 



