244 



VEKTEBRATE ANIMALS. 



throughout life, and by having an integumentary skeleton composed 

 of ganoid scales, plates, or spines (fig, 164, d). The skull is com- 

 posed of distinct bones, and there is always a lower jaw. There are 

 usually two pairs of fins (pectoral and ventral), supported by many 

 series of cartilages, and the ventral fins are placed very far back. 

 The first rays in the fins are usually in the form of strong spines. 

 The caudal fin or tail is mostly heterocercal or unsymmetrical (fig. 

 167, B). The swim-bladder is always present, is often cellular, and 

 is pi'ovided with an air-duct. The gills and gill-covers are essen- 

 tially the same as in the Bony fishes. The heart has one auricle 

 and a ventricle ; and the hulhiis arteriosus is rhythmically contrac- 

 tile, is furnished with a distinct coat of muscular fibres, and is 

 provided with several transverse rows of valves. 



The best known of the living Ganoids are the Bony Pike or Gar- 



Fig. 175. — A, LcpUhisti'its nssi'u^, the "Gar-Pike" of tlie American Lakes; B, Aspido- 

 rliynch-us, restored (after Agassiz), a Jurassic Ganoid allied to Lepidosteiis, but 

 having a honiocercal tail. 



fish {LepidosU'iis), the Sturgeon {Acipenser), and the Poh/ptenis. Of 

 these, the Bony Pike (fig. 17'), A), is found in the rivers and lakes 

 of North America. It is n large fish, attaining a length of several 

 feet, and it lias the body entirely covered with aii armour of ganoid 

 scales arranged in "Ijliquely transverse rows, The jaws form a long 

 narrow snout, armed with a double series of teeth, and the tail is 

 heterocercal. The vertebral column is more perfectly ossified than 

 in any other fish, the bodies of the vertebr;e being convex in front 

 and concave behind {'' opistJinrrdovs"). The Pott/ptvrvs inhabits the 

 rivers Nile and Senegal, and is remarkable for the peculiar structure 

 of the dor.s.MJ fin, which is broken up into .a series of small detached 

 pjortions, each com] wised of a .single sjiine in front, with a soft fin 

 attached to it behind. Some of the species of Polypicrvs possess 



