370 ZOOLOGY. 
From the underside of the vertebre 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, 
Spe 
Gi 
Fig. 366.—Transverse section of a worm, of Amphioxus, and of a Vertebrate con- 
trasted. a, outer or skin layer; }, dermal connective layer; c, muscles; d, seg- 
mental organ; 2, arterial, and 2, venous blood-vessel; g. intestine ; /, notochord.— 
After Haeckel. 
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 onlyin 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 vertebra ; 
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 vertebra of the early em- 
bryos of all Vertebrates. 
The digestive canal consists of a mouth with lips or jaws, 
armed with teeth, a pharynx leading to the lungs ; an cesoph- 
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 
