44 ASCIDIANS 



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, sg). 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 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 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. 83) 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 : — 



(a) As of the same nature as the blood spaces (blastocoelic), or 

 (h) 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 



