140 



MMT LBBSOm IN ZOOLOGY. 



side of the pharynx. The blood-corpuscles are white and 

 nucleated. 



The vertebral column is represented by a flexible gristly 

 rod called the " notocordj" which extends to the end of the 

 head far in front of the nervous cord, which lies over the 

 notocord. The nervous cord is not divided into a true brain 

 (though there are faint traces of one) and spinal cord, but 

 sends ofE a few nerves to the periphery, with a nerve to the 

 single minute eye-spot. 



We see, then, that though the lancelet is at the bottom 



Fio. 146.— The lancelet. a, vent; /, stomach; g, pharynx: m, nervous cord; p, 

 pore; r, notocord; *, tentacles around the mouth. (Enlarged twice.) 



of the vertebrate scale, yet it has the most fundamental 

 vertebrate marks, that of a rudimentary backbone, i.e., a 

 notocord, with the nervous system placed above, and the 

 other internal organs below. 



But there is a group of animals which partly bridge over 

 the gap between the lancelet and the worms. 



These are the sea-squirts or ascidians (Fig. 147). One 

 of them, called Appendicularin , is like a tadpole in general 

 appearance, while the larvae of most of 

 them are tadpole-shaped, as in Fig. 148. In 

 the infant ascidian the tail is supported by 

 a gristly rod {u), extending into the chest, 

 and corresponding to the notocord of the 

 lancelet. Above it is the nervous cord («), 

 and in the head is an eye {(>') and an ear (o), 

 * AnascSan'cS ^l^ile the mcuth (.-) opens into a pharynx, 

 uraisize) which in after-life has gill-slits. Towards 



adult life the ascidians (perhaps except Appendicularia) 

 take a backward path in their development, and lose the 

 startling vertebrate features of their youth. Like some 

 precocious human children, they cease to fulfil the promise 



