186 



ZOOLOGY 



SECT. 



FIG. SOS. Scale of Salmo fario. 



a. anterior portion covered by 

 overlap of preceding scales ; 



b, free portion covered only by 

 pigmented epidermis. 



-N.SP 



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

 portion (b) 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. 



En do skeleton. --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 vertebrae. 

 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 bv 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 vertebrae, by ankylosis 

 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 vertebrae they are attached directly to the centrum, in the 



PA.PH 



l-'ic. 801'. Salmo fario. A, one of the 

 anterior, and B, one of the posterior ab- 

 dominal vertebra? ; 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. rib. 



