VERTEBRATA 



PHYLUM VIII 



Amphibia, Beptilia, Aves, and Mammalia. Recently the numerous groups of 

 Tunicata and Leptocardii have been placed with these as special classes and 

 often regarded as the ancestors of the Vertebrates. As these have left no 

 fossil remains in the rocks, the palaeontologist discovers no clue to the origin 

 of the Vertebrata. 



Class 1. PISCES. Fishes. 



Cold-blooded animals living in water and usually breathing exclusively by gills. 

 Limbs in the form of fins. Skin with scales or bony plates, rarely naked. Vertebral 

 axis ending in a vertical caudal fin. Heart with a single ventricle and auricle. No 

 amnion or allantois. 



To the skin structures of fishes belong the scales, dermal bones, spines, 

 fin rays, and teeth. 



Scales. — Louis Agassiz distinguished four kinds of scales, to which in his 

 system there correspond as many orders. 



1. Placoid scales occur only among the Selachians. They are usually 



small rhombic plates, star- 

 shaped, 1 e a f - s h a p e d, 

 pointed, spade -shaped, or 

 even conical structures 

 which lie close together 



and form a rough mosaic 



A 



B 



A, Scale 

 Recent. 5 '/j 

 (I'riui/ndon) 

 Magnified. 



Fig. 1. 



of Set/Ilium fa itU- ula. 

 B, Scales of CarcJiarias 

 ganged cus. Recent. 



Fig. -2. 



Large placoid scale of Raja 

 antiqua, with spine. Pliocene. 

 Nat. size. 



(shagreen). The placoid 

 scales are often of variable 

 form on different parts of 

 the body, and sometimes 

 (e.g. among the rays) single 



tubules are conspicuous by their large size and sculpture, and appear as thick 



plates externally roughened 



or provided with spines. 



Placoid scales (Figs. 1-3) 



have the structure of teeth 



and consist of a basal plate 



(of thick phosphate of lime) 



penetrated by vertical 



bundles of connective tissue 



fibres sunk in the skin, and 



a freely projecting upper 



part composed of dentine 



and penetrated by coarser 



and finer little canals. All 



the canals arise from a cavity 



(pulpa, p) filled with connec- 

 tive tissue and tooth cells 



(odontoblasts, o), and sub- 

 divided outwards into continually finer branches. Besides phosphate of lime 



the dentine comprises small quantities of fluoride and carbonate of lime. The 



core of dentine is covered by a thin, shining, very hard structureless outer 



layer, which corresponds histologically and chemically with the enamel of 



Sagittal section through a scale of Scymuus lichia. Recent. 60/j. 

 B, Basal plate. D, Dentine. S, Enamel ; c, Cutis ; d, Horizontal 

 dentine canals ; /, Bundles of connective tissue ; h, Large principal 

 dentine canal extending upwards; o, odontoblasts; p, Pulp cavity 

 (after O. Hertwig). 



