GENERAL NOTES ON TUNIC ATA. 



45' 



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

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

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

 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 in- 

 vaginations form the original 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, 



FIG. 243. Embryo of Cla-veHna. Modified after Seeliger. 



f.p. t Fixing papilla; e*f. t ectodermic fold; c.g.> ciliated groove; 

 en.) endostyle ; s.o.j cerebral vesicle with sense organs; g".s. t 

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

 chord ; -., gut curving upwards towards atrial opening. The 

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

 atrial opening are indicated by arrows. 



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 

 single atrial aperture of the adult. The gill-slits increase greatly in 

 number, the increase being due both to the formation of new slits and 

 to the division of those first formed, and the whole animal under- 

 goes a metamorphosis which is one of the most signal instances of 

 degeneration. 



GENERAL NOTES ON TUNICATA 



The description of Ascidia given above is, in its general 

 outlines, applicable to all the simple Ascidians, which are 



