GENERAL NOTES ON TUNICATA. 451 
Beneden and Julin, the mesoderm primarily originates in the form of 
two pockets, which grow out from the gut, as in. Amphioxus, and 
whose cavity is the true ccelom. 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 kours the larva enjoys a free-swimming life, using its tail 
as an organ of locomotion. Then it fixes itself by papillee on its head, 
& /P 
Fic, 243.—Embryo of Clavelina.— Modified after Seeliger. 
J#-, Fixing papilla; ef£, ectodermic fold; c.g., ciliated groove; 
en., endostyle; s.o., cerebral vesicle with sense organs; g's., 
gill-slits ; May nerve-cord beginning to degenerate; ch., noto- 
chord; g., gut curving upwards towards atrial opening. The 
atrial invagination is marked bya 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 2 metamorphosis which is one of the mast 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 
