BONY FISHES AND GANOIDS. 463 



number of minute eggs, crowded together into a mass. Whereas in the 

 higher types the optic nerves simply cross one another, and there is no spiral 

 valve to the intestine, in the ganoids the latter is present, and the fibres of 

 the optic nerves interlace where they meet, to form what is known as a 

 chiasma by the complete fusion of the two nerves. Very generally an air- 

 bladder is present, and the less specialised bony fishes and all the ganoids 

 are provided with a duct to the gullet. In the higher types the rays of the 

 fins are entirely of dermal origin, their cartilaginous supports being abbre- 

 viated and enclosed within the body. In addition to the features already 

 noticed, the ganoids have a larger number of gill-arches, a spiracle, an 

 arterial cone furnished with many valves to the heart, a cellular air-bladder, 

 and several shark-like features in other organs. 



ORDER I. ACTINOPTERYGII. 



SUB-ORDER I. ACANTHOPTERYGII. 



The first great order of the sub-class includes what may be termed the 

 Fan-finned Teleostomes, in which the fins are entirely supported by dermal 

 rays, the primitive cartilaginous supports being greatly reduced and enclosed 

 in the body-wall. A single paired series of transversely elongated rays to 

 which an unpaired anterior element .may be added are developed in the 

 branchiostegal membrane between the two branches of the lower jaw. The 

 caudal fin is variable. The spiny-finned fishes, forming the sub-order Acan- 

 thopterygii, together with the six following sub-orders, 1 are collectively dis- 

 tinguished by the supporting bony elements of the dorsal and anal fins being 

 equal in number to the dermal rays, and by the tail being generally of the 

 homocercal or abbreviate-heterocercal type although occasionally diphy- 

 cercal. In the Acanthopterygii and four following sub-orders the skeleton is 

 fully ossified, the optic nerves simply cross, and there is no spiral valve to 

 the intestine. In common with the three following sub-orders, they have 

 generally no duct to the air-bladder, the two parietal bones of the skull 

 separated by the supra-occipital, and the pelvic fins usually jugular or 

 thoracic. The chief distinction of the Actinopterygii themselves is that usu- 

 ally some of the rays of the dorsal and anal fins are not articulated, and form 

 spines. 



In this section the lower pharyngeal bones are generally distinct, and the 

 scales ctenoid ; while the preopercular bone is not connected by a bony stay 

 with the orbit, the spinous dorsal is large, and the pelvic fins 

 are thoracic, and usually furnished with five branched rays, Section 1. Perci- 

 although sometimes with only four. Certain other features formes. Family 

 connected with the skeleton are too complex to be noticed Centrarchidf. 

 here. The Centrarchidce form a comparatively small family, 

 containing ten genera, and typified by the single species of Centrarchus, from 

 the fresh waters of the United States ; one of their distinctive characters 

 being the presence of three or more spines in the anal fin. In habits these 

 fishes are carnivorous ; and many of them resemble the stickle-backs in 

 building nests. 



lr The following are the sub-orders of the Actinopterygii -viz., (1) Acanthopterygii, (2) Lopho- 

 branchii, (3) Plectognathi, (4) Anacanthini, (5) Physostomi, (6) Jitheospoudyli. (7) Protospcndyli, 

 (8) Chondrostei. 



