44 ASCIDIANS CHAP. 
opens to the exterior dorsally by the atrial aperture, forms the 
cloaca along the dorsal edge of the body, and has extensions 
laterally on each side of the branchial sac, with the interior of 
which it is placed in communication by the secondary gill-slits 
or “stigmata” (Fig 19, sy). Along the ventral edge the mantle 
is united to the wall of the branchial sac, and it is only this 
union (Fig. 19, end) that prevents the peribranchial cavity from 
completely surrounding the branchial sac. 
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 sae. 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 outgrowth 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 (blastocoele) 
remains, where not obliterated by the development of the meso- 
dermal connective tissue, as the irregular system of blood spaces, 
with its outgrowths in test and branchial sac. The heart, which 
has differentiated muscular walls, becomes secondarily connected 
at its ends with these blood spaces. 
4. The pericardium and epicardium (see p. 85) originate as 
outgrowths from the archenteron. They may therefore be re- 
garded as enterocoelic spaces. The pericardium becomes com- 
pletely closed off and separated from the alimentary canal. The 
epicardium may form paired tubes of great length, and may 
remain permanently connected with the branchial sac. 
5. The cavities of the renal vesicles and of the gonads and 
ducts are spaces formed in the mesoblast. They have been 
variously interpreted :— 
(aw) As of the same nature as the blood spaces (blastocoelic), or 
(>) As formed by a splitting of the mesoblast (coelomic). 
6. The cavity of the neural gland and of its duct opening at 
the dorsal tubercle is derived from the primitive dorsal neural tube 
of the embryo, and so may be regarded as a part of the lumen of 
the cerebro-spinal nervous system. 
Tentacles, etc.—The branchial aperture leads through the 
