186 



ZOOLOGY 



SECT. 



FIG. 808. Scale of Salmo fario. 



a. anterior portion covered by 

 overlap of preceding scales ; 



b, free portion covered only by 

 pigmented epidermis. 



like the tiles of a house, in such a way that a small three-sided 



portion (&) of each scale comes to lie immediately beneath the 



epidermis, while the rest (a) is hidden 

 beneath the scales immediately anterior 

 to it. Besides the scales, the fin-rays 

 belong to the exoskeleton, but will be 

 most conveniently considered in con- 

 nection with the endoskeleton. 



Endoskeleton. The vertebral 

 column shows a great advance on that 

 of the two previous classes in being 

 thoroughly differentiated into distinct 

 bony vertebrae. It is divisible into an 

 anterior or abdominal region and a 

 posterior or caudal region, each con- 

 taining about twenty-eight vertebra?. 

 A typical abdominal vertebra consists of a dice-box-shaped 



centrum (Fig. 809, CN.) with deeply concave anterior and posterior 



faces, and perforated in the centre by a small hole. The edges of 



the centra are united by liga- 

 ment and the biconvex spaces 



between them are filled by the 



remains of the notochord ; there 



are also articulations between 



the arches by means of little 



bony processes, the zygapophyses 



(x. ZYG., H. ZYG.). To the dorsal 



surface of the centrum is at- 

 tached, by ligaments in the 



anterior vertebras, by arikylosis 



or actual bony union in the 



posterior, a low neural arch 



(x. A.), which consists in the 



anterior vertebra? of distinct 



right and left moieties and is 



continued above into a long, 



slender, double neural spine 



(x. SP.), directed upwards and 



backwards. To the ventro- 



lateral region of the vertebra 



are attached by ligament a 



pair of long, slender ribs (R.) 



with dilated heads, which curve 



downwards and backwards be- 

 tween the muscles and the 



peritoneum, thus encircling the abdominal cavity. In the first 



two vertebras they are attached directly to the centrum, in the 



FIG. 809. Salmo fario. A, one of the 

 anterior, and B, one of the posterior ab- 

 dominal vertebrae ; C, one of the anterior, 

 and D, one of the posterior caudal vertebra}. 

 CN. centrum; 1MB, inter-muscular 

 bone; HA. haemal arch ; H. SP. haemal 

 spine; H. ZYG. haemal zygapophysis ; 

 N. A. neural arch ; N. SP. neural spine ; 

 N. ZYG. neural zygapophysis ; PA. PH. 

 parapophysis ; R. ril 



