140 



FIRST LESSONS 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 "notocord," 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 off 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 



FIG. 146. The lancelet. , vent; /, stomach; <7, pharynx; n, nervous cord; p, 

 pore; r, notocord; f, 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 Appcndicularia, 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 (n), extending into the chest, 

 and corresponding to the notocord of the 

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

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

 while the mouth (*) opens into a pharynx, 

 which in after-life has gill-slits. Towards 

 adult life the ascidians (perhaps except Append icularia) 

 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 



