90 



LOWER VERTEBRATES. 



Class Y. — PISCES. 



Comparatively recent investigations show that the old group of fishes, the Pisces 

 of the older systematic works, needs to be dismembered. Forms like Amphioxus and 

 the lampreys are fully as different from the ordinai-y types of fishes as are the Batraehia 

 from the reptiles, or the latter from the birds. But exactly where or what lines shall 

 be drawn, science has not finally determined. For the present, however, it seems best 

 to restrict the term fishes or Pisces (its Latin equivalent) to the series of forms em- 

 bracing the great majority of forms now living, which have the following characters 

 in common : — 



The skeleton may be either cartilaginous or hardened by the deposition of salts of 

 lime in its tissues. The skull is well developed, the bones or elements of which it is 

 composed are united by sutures, and a lower jaw is present. The gills are normally 

 four in number, and are borne on bony arches. The water, after passing though the 

 mouth and over the gills, goes out through a single slit on either side. The body 

 is usually covered with scales, and on the heads of some membrane-bones are devel- 

 oped. Both median and 

 paired fins are usually pres- 

 ent. Normalh', and in the 

 young, the foi-mer forms a 

 continual fin around the 

 body, beginning behind the 

 head, jiassing back around 

 the tail, and forward on the 

 ventral surface as far as the 

 vent ; but usually it is, in the adult, broken up into several portions, known, according 

 to their position, as dorsal, caudal, and anal fins. The paired fins are always in 

 two pairs, corresponding to the two pairs of limbs of the higher vertebrates. Those 

 representing the fore limbs are called the pectoral, those homologous with the hind legs 

 are known as the ventral fins. The brain is larger than the rest of the nervous system, 

 and is divided into three portions, the fore, mid, and hind brains. A well-developed 

 heart is present, consisting of an auricle, a ^■entricle, and a IiuUjous arteriosus, the 

 latter having a varying number of valves in its interior. 



Of the Pisces, two divisions are usually recognized, the ganoids and the tele- 

 osts. In some respects one, in others the other, of these seems to be the more 

 highly differentiated, though it is evident that the lattei- have deviated the more 

 widely from the jirimitive stock from which the other types of vertebrates have arisen. 

 With this uncertainty before us, it is of little importance which we take up first, and 

 so, from convenience, we begin with the Ganoidea. 



Fig. 67. — Fins of a fish; n. anal; c, caudal; rf, dorsal; /), pectoral; r. ve 

 (The first three are known as median, the others as paired fins.) 



Sub-Class I. — Ganoidea. 



The ganoids receive their name from a Greek word meaning s])lendor, in allusion 

 to the enamelled armor with which most of them are encased. As first defined it em- 

 braced several forms now recognized as teleosts, a mistake arising from the fact that 



