390 



ZOOLOGY. 



From the underside of the vertebrae are sent off processes 

 articulating with the ribs, whicli 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 jiairs of limbs, supported by rays ir- 

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



Fig. 371.— Transverse section of a worm, of Amphioxus, and of a Vertebrate con- 

 trasted. «, outer or skin layer; b, dermal connective layer; e, muscles; d^ seg- 

 mental organ : A, arterial, and i, venous blood-vessel ; g. intcstiue ; t, notochord. — 

 -Vf ter 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 p)rom- 

 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 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 pirimitive vertebrae of the early em- 

 bryos of all A^ertebrates. 



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

 armed with teeth, a pharynx leading to the lungs ; 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 pass 

 ducts leading from a large liver, usually provided with a gall- 

 bladder, a pancreas, and a spleen. The products of digestion 

 do not pass through the walls of the stomach and directly 

 enter the circulation, as in the invertebrates, but there is a 



