a 
46 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
4. The pericardium and epicardium originate as out- 
erowths from the archenteron. They may therefore be 
regarded as coelomic spaces. The pericardium becomes 
completely 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 in some Tunicata. 
5. The cavities of the renal vesicles and of the gonads 
and ducts are spaces formed in the mesoblast. They have 
been variously interpreted :— 
(a) As of the same nature as the blood spaces (blasto- 
coelic), or 
(b) As formed by a splitting of the mesoblast (coelomic). 
6. The cavity of the neural gland and 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. 
BRANCHIAL SIPHON AND TENTACLES. 
The branchial aperture opens into a large cavity, the 
Branchial Sac (PI. II., fig. 4, b7.s.), which is merely the 
anterior portion of the alimentary canal, corresponding 
to the pharynx or back of the mouth in man, enlarged and 
greatly modified so as to act as a breathing organ, or 
branchia—whence its name—in addition to performing 
other important functions. 
The branchial aperture itself (B7.) is thus the mouth ot 
the Ascidian, and the siphon is therefore the commence- 
ment of the alimentary canal. Its inner surface is lined 
for a short distance by a prolongation of the test, and ~ 
where this stops, at about the line of junction of the 
ectoderm of the stomodcum with the endoderm of the 
mesenteron, a circle of delicate hair-lke Tentacles (Pl. IL., 
