204 UROCHORDA Ai\D VERTEBRATA 



apertures to the bod}', one for the ingress antl the other for 

 the egress of water. From the latter opening the water is 

 forced in a stream by the contraction of the muscular mantle. 

 This habit gives them the conmion namo, sea squirts. 



Most of the ascidians possess a notochord only in the 

 larval stage, losing it entirely as they become atlults. 



VERTEBRATA 



Fishes, frogs, reptiles, birds, and mammals. — By far the 



greater number of animals belonging to the branch Ghor- 

 data possess a spinal cohunn, or backbone made up of 

 bony segments called vertelirse, — hence the name verte- 

 brates. The animals named at the beginning of this 

 paragraph are familiar examples of vertebrates. The body 

 of a vertebrate is divided into three i-egions: head, trunk, 

 and tail. When limbs are present, there arc never more than 

 two pairs. In general, vertebrates have an internal skele- 

 ton. Some have also a partial external skeleton, as croco- 

 diles, tortoises, and others, while many possess vestiges of an 

 external skeleton represented by nails, horns, hair, feathers, 

 hoofs, etc. Among vertebrates we find t\A'o kinds of respi- 

 ratory organs, namely, lungs and gills. The land-living 

 vertebrates and many of those living in the water are fur- 

 nished with lungs, but the fishes and a few of the amphibians 

 possess gills for lireathing. In all except the lancelet 

 there is a single contractile cavity, or heart. In all there 

 is a system of blood vessels for the circulation of the blood. 



Lancelet 



C/o.s-.s'. — Acnniia 



Lancelet. — The lancelet is the lowest vertebrate. It 

 lives the life of a fish and somewhat resembles a fish in form. 



