10 



4. The pericardium and epicardium originate as out- 

 growths 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. 



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 

 (h) 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, hr.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 (Br.) 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 stomodoeum with the endoderm of the 

 mesenteron, a circle of delicate hair-like Tentacles (PI. II., 



