the antero-dorsal part of the body, where t. indicates the 

 test, and spli. the sphincter of the branchial aperture. The 

 cavity marked p. hr. 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 ill 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 Body. 



The following list of the cavities present in the body of 

 the adult Ascidia maj'' be useful at this point : — 



1. The alimentary canal, including the branchial sac. 

 This is derived from the archenteron of the embryo, is 

 lined throughout by endoderm, and the system of cavities 

 of the intestinal gland is to be regarded merely as an out- 

 growth 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. 



