ASCIDIA. 45 
the antero-dorsal part of the body, where ¢. indicates the 
test, and sph. the sphincter of the branchial aperture. The 
cavity marked p. br. is a portion of the atrial cavity lined 
by ectoderm, and not to be confounded with a coelom or 
body-cavity. The absence of a true coelom in the 
mesoderm will be noticed in this and the other figures 
(e.g. fig. 4); and yet the Tunicata are Coelomata—although 
it is very doubtful whether an enterocoele is ever formed, 
as has been described by E. van Beneden and Julin in the 
development of some. The primitive coelom is, however, 
largely suppressed during development, and is only repre- 
sented in the adult by the pericardium and small cavities 
in the renal and reproductive organs and ducts, as will be 
shown further on. 
CAVITIES OF THE Bopy. 
The following list of the cavities present in the body of 
the adult Ascidia may be useful at this point :— 
1. The alimentary canal, including the branchial sac. 
This is derived from the archenteron of the embryo, is 
lmed throughout by endoderm, and the system of cavities 
of the intestinal gland is to be regarded merely as an out- 
erowth from the alimentary canal. 
2. The peribranchial (atrial) cavity, derived from two 
lateral ectodermal invaginations which join dorsally to 
form the cloaca and open to the exterior by the atrial 
aperture. 
3. The original embryonic segmentation cavity (blasto- 
coele) remains, where not obliterated by the development 
of the mesodermal connective tissue, as the irregular 
system of blood spaces, with its outgrowths in test and 
branchial sac. The heart is only a special part of this 
cavity which has differentiated muscular walls, 
