370 



ZOOLOGY. 



From the underside of the vertebrae are sent off processes 

 articulating with the ribs, which enclose the digestive and 

 central circulatory organs. There is a skull formed by a con- 

 tinuation of the vertebral column, enclosing a genuine brain, 

 consisting of several pairs of ganglia. To the vertebral col- 

 umn are appended two pairs of limbs, supported by rays ir- 

 regularly repeated, or a series of bones of a definite number, 



Fig. 3uti. Transverse section of a worm, of Amnhioxus, and of a Verti brate con- 

 trasted. , outer or skin layers 6, dermal connective layer; <:. musi-le* ; d, seg- 

 mental organ : h, arterial, and i, venous blood-vessel ; g. intestine ; I, uotochord. 

 Aiter llaeckel. 



attached to the vertebral column by a series of bones called 

 respectively the shoulder and pelvic girdle. 



It will be observed that the fact of segmentation, so prom- 

 inent a feature in the Worms and Arthropods, survives, or at 

 least reappears in a marked degree in the Vertebrates, as 

 seen not only in the vertebral column, but in the arrangement 

 of the spinal nerves. It is perceived also in the arrangement 

 of the muscles into masses corresponding to the vertebrae ; 

 and in the segmental organs or tubes forming the kidneys of 

 the sharks and rays, while segmentation is especially marked 

 in the disposition of the primitive vertebras of the early em- 

 bryos of all Vertebrates. 



/ 



The digestive canal consists of a mouth with lips or jaws, 

 firmed with teeth, a pharynx leading to the Jungs ; an oesoph- 

 agus and thyroid gland ; sometimes a crop (ingluvies), often 

 a fore-stomach (proventriculus) ; a stomach and intestine, 

 cloaca and vent. Into the beginning of the intestine passes 

 a duct leading from a large liver; a gall-bladder, usually a 

 pancreas, and a spleen, also communicating with the intestine. 

 The products of digestion do not all pass through the walls 

 of the stomach and directly enter the circulation, as in the 

 invertebrates, but there is a system of intermediate vessels 



