CHAPTER LIII 



CLASS PISCES 



The bony fishes, which form the third class of vertebrates, Pisces, 

 include a large assemblage of types of an exceedingly varied character. 

 They possess the following characteristics: (1) The skeleton is more 

 or less bony, resulting from replacement in different degrees of the 

 primitive cartilaginous skeleton. (2) The gills on each side are covered 

 and protected by a fold, or operculum, supported by dermal bones. 

 (3) The pelvic girdle is usually small or absent. (4) The fins are sup- 

 ported by fin rays. (5) The dermis contains scales of different types, 

 but in no case are they placoid. (6) Most of the bony fishes have an 

 air bladder. (7) The brain includes a small cerebrum, very small 

 olfactory lobes, and well-developed optic lobes and cerebellum. Prac- 

 tically all of these characteristics present a contrast to those of the 

 elasmobranchs. 



387. Classification. — The class Pisces may be divided into two 

 subclasses — Teleostomi (tel e 6s' to mi; G., teleos, complete, and stoma, 

 mouth), or bony fishes proper, and Dipnoi (dip' no i; G., dipnoos, with 

 two breathing apertures), or lungfishes. There are four divisions of 

 Teleostomi: (1) Crossopterygii (kro sop ter ij' i i; G., krossoi, fringe, 

 and pterygion, fin), or lobe-finned ganoids; (2) Chondrostei (kon dros' te i; 

 G., chondros, cartilage, and osteon, bone), or cartilaginous ganoids; 

 (3) Holostei (holos'tei; G., holos, whole, and osteon, bone), or bony 

 ganoids; and (4) Teleostei (tel e 6s' te T; G., teleos, complete, and osteon, 

 bone), which includes the common bony fishes and far exceeds in number 

 of species all of the other divisions combined. 



368. Crossopterygii. — The lobe-finned ganoids were very abundant 

 in the Devonian period, twenty million years or more ago. Because 

 fishes were the dominant form of animal life, this period is known as 

 the age of fishes. The Crossopterygii, however, are now represented 

 by only two types, both found in Africa. There are evidences that 

 this group is ancestral not only to all higher fishes but also to the terres- 

 trial vertebrates. A fact which indicates the last relationship is the 

 existence of a larva which is very similar to the tadpoles of Amphibia. 

 Both the larva and the adult use the pectoral fins as supporting append- 

 ages. The air bladder in Polypterus (Fig. 232), one of the two living 

 types, is used not only as a hydrostatic organ but also as an accessory 

 respiratory organ, being connected by a primitive trachea with the 

 pharynx and used as a lung. 



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