VERTEBRATA 



PHYLUM VIII 



Amphibia, jReptilia, 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. 



Fig. 1. 



A, Scale of ScijUium canicula. 

 Recent. 5J/j. j}^ Scales of Carclmrias 

 (PHonodon) gangetictts. Recent. 

 Masnitied. ' 



Fig. 2. 



Large placoid scale of liuja 

 antiqna, with spine. Pliocene. 

 Nat. size. 



Glass 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 eiuling 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 

 J ^ small rhombic plates, star- 



shaped, leaf -shaped, 

 pointed, spade -shaped, or 

 even conical structures 

 which lie close together 

 and form a rough mosaic 

 (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 b}'' 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 Iw coarser 



and finer little canals. All 



the canals arise from a cavity sagittal section through a scale of Scynmus Uchia. Recent. «>«/i. 



, , \ en J -1, B, Basal plate. D, Dentine. S, Enamel; c, Cutis; d, Horizontal 



(pulpa, p) nllect with COnneC- dentine canals ; /, Bundles of connective tissue; h. Large principal 



tive tissue and tooth cells J™^;"^/^",^^,!;^;'"^'"^' "p^^^^^^ 

 (odontoblasts, o), and sub- 

 divided outwards into continually finer branches. Besides jDhosphate 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 



