244 



VERTEBRATE ANIMALS. 



throiighriut life, and by having an integumentary skeleton composed 

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

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

 usuidly two pairs of tins (pectoral and ventral), su]iported by many 

 series of cartilages, and the ventral fins ai-e placed very far back. 

 The hrst rays in the tins are usually in the form of strong spines. 

 The caudal hn or tail is mostly heterocercal or unsynimetrical (fig. 

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

 is provided 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 ventiicle ; and the hnlhiis arteriosus is rhythmically contrac- 

 tile, is furnished with a distinct coat of niuscuhu' fibres, and is 

 provided with several transverse rows of valves. 



The best known of the living (4anoids are the Bony Pike or Tiar- 



Fig. 17n.— A, lfi>i(h,itrus nsMHs, tlm " Gar-Pike" nf tlie American Lakes; B, AspiOo- 

 rhiim^hti^, rcstnreil (altcT' Agassiz), a .JuraSf^ic Ganoid allied to Lepldosteus, but 

 havini; a h. -i-rral tail. 



fish {Lcpii.losteAts), the Sturgeon {Aripeiixcr), and the I'uli/pterns. Of 

 tlie.se, the Bony Pike (fig. 17.'>, A), is fcmnd in the rivers and lakes 

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

 feet, and it has the ImkIv eidiiely covered with an ai'iuovu- of ganoid 

 .scales arranged in oblicpiely transverse rows. The jaws form a long 

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

 heteioceieal. The vertebi"il (aihiiun is more perfectly ossified than 

 in any other fi.sh, the bodies of the veilelir.e being convex in front 

 .■uid concave beldnd ('' iipixlliiiri^liiiix" ). The Pn/i//ifrnix inhabits the 

 rivers Nile .-ind Seneg;d, .■lud is remarhalile for the ]ieeuliar structure 

 of the dorsal fin, \\hirli is binkoi up into a .series of small detached 

 jiortions, each coui|iosed of a, single sjiine in front, with a .scift fin 

 .■i.ttaclicd (o it lieliind. 8ome of the species of /'n/i/jitrni.i possess 



