432 



SUB- PHYLUM URQCHORDA OR TUNIC AT A. 



\Yith regard to the origin of mesoblast and notochord, there is more 

 difficulty. Both originate from the endoderm in the region of the 

 blastopore, and for a time grow forward together. The notochord lies 

 in its usual position on the roof of the gut, from a specialisation of which 

 it arises ; but its forward extension is limited, it never extends into the 

 anterior region, and in the posterior region the future tail it increases 

 at the expense of the primitive gut, whose lumen it obliterates. The 

 mesoderm, on the other hand, extends right forward, and becomes 

 divided into two regions a posterior, ultimately forming the muscula- 

 ture of the tail, and an anterior, giving rise to the blood, connective 

 tissues, body muscles, excretory and genital organs. According to Van 

 Beneden and Julin, the mesoderm primarily originates in the form 01 

 two pockets, which grow out from the gut, as in Amphioxus, and 

 whose cavity is the true crelom. According to the majority of investi- 



n 



FIG. 212. Embryo of Clavelina. Modified after Seeliger. 



f.p.. Fixing papilla; e.f., ectodermic fold; e.g., ciliated groove; 

 en., endostyle ; s.c., cerebral vesicle with sense organs; g.s., 

 gill-slits; ., nerve-cord beginning to degenerate; ck., noto- 

 chord ; g:, gut curving upwards towards atrial opening. The 

 atrial invagination is marked by a dotted line ; the mouth and 

 atrial opening are indicated by arrows. 



gators, it originates as solid blocks of cells, and the body cavity is only 

 represented by spaces produced by the subsequent separation of these 

 cells. 



The further processes of development result in the formation of a 

 tadpole-like larva, with dorsal nervous system, notochord in the tail 

 region, and well -developed sense organs. Two ectodermal invagina- 

 tions form the originally double peribranchial chamber, and small 

 diverticula from the pharynx meet these and form the first gill- 

 slits. 



For some hours the larva enjoys a free-swimming life, using its tail 

 as an organ of locomotion. Then it fixes itself by papillae on its head, 

 and begins almost immediately to degenerate. The tail shrinks and 

 disappears, being consumed by phagocytes. The nerve-cord is lost, 

 and with it the larval sense organs, while simultaneously a change of 

 axis results in the adult relation of parts. The peribranchial chamber 

 becomes greatly enlarged, and its two openings fuse together to form the 



