FREE ASCIDIAN LARVAE. 



457 



On making a section through the middle of the body in 

 this stage (at the point where the tail joins the trunk), we 

 find in the Ascidian larva precisely the same characteristic 

 disposition of the chief organs as in the larva of the 

 Amphioxus (Plate X. Fig. 6). In the middle, between the 

 medullary tube and the intestinal tube, is the chorda dor- 

 sal is; and on each side of the latter, the muscle-plates of 

 the back. The section of the Ascidian larva now differs in 

 no essential way from that of our ideal Vertebrate (Fig. 

 161). 



When it has reached this stage of development, the 

 Ascidian larva begins to move within the egg-covering. 

 This ruptures the egg-covering ; the larva emerges from the 

 latter, and swims freely about in the sea by means of its 

 rudder-like tail (Plate X. Fig. 5). These free-swimming 

 Ascidian larva have long been known to science. They 

 were first observed by Darwin during his voyage round the 

 world in 1833. In external form they resemble the larva 

 of the frog, the tadpole, and they move about in the water 



Fi<5. 161. — Transverse section through ideal 

 Primitive Vertebrate (Fig. 52). The section 

 passes through the sagittal axis and the cross 

 axis : n, medullary tube ; x, notochord ; t, dorsal 

 vessel ; v, ventral vessel ; a, intestine ; c, body- 

 cavity ; ro„ dorsal muscles; m 2 , ventral mus- 

 cles ; h, outer skin. 



like the latter, using their tail as a rudder. This highly 

 developed youthful condition of free movement lasts, how- 

 ever, only for a short time. A further progressive develop- 



