408 TUNICATA. 



the adult Ascidiozooid. The upper surface of the body in the Ascidio- 

 zooid thus contains one half of the later dorsal side and the corre- 

 sponding half of the later ventral side. The boundary between these 

 two halves is marked by the position of the branchial aperture (Fig. 

 201 A, >'). In comparing the adult with the young Ascidiozooid, we 

 must bear in mind the distinctions, which may be tabulated as 

 follows : — 



YOUNG. ADULT. 



Upper surface of the body. = Surface of the anterior half of the 



body. 



Lower surface of the body. = Surface of the posterior half ol the 



body. 



The anterior part of the embryo from = Neural surface of the body, 

 the branchial aperture to the corre- 

 sponding point of the lower side. 



The posterior part of the embryo, from - Haemal surface of the body (marked 

 the branchial aperture to the corre- by the endostylel. 



sponding point on the lower side. 



The terms " right " and " left," however, are applicable to the 

 same sides of the body in the young and the adult. 



As the gill-slits increase in number and in size, the peribranchial 

 sacs, the epithelium of which flattens, enlarge coiTespondingly. Each 

 of them gives off a diverticulum to the lower surface of the body 

 which, by fusing with a corresponding diverticulum from the other 

 side, leads to the formation of an unpaired atrial cavity (Fig. 200, cl). 



The branchial aperture (Fig. 200, i) is derived from an ectodermal 

 invagination on the upper surface of the embryo between the nervous 

 system and the anterior end of the endostyle. We cannot here enter 

 further into the somewhat complicated processes which, according to 

 Salensky, lead to the development of this aperture and its valve- 

 like closing apparatus. 



The branchial aperture appears at a rather early stage, but the 

 exhalent or atrial aperture only forms at a late stage after the 

 development of the Ascidiozooid is, in other respects, completed. 

 The common cloacal cavity of the colony, into which the atrial 

 apertures of the individuals open, was traced back by Kow alrysky 

 to the atrial cavity of the Cyathozooid. Salensky on the contrary, 

 observed that, when the Cyathozooid degenerates, its atrial aperture 

 closes, and consequently atrophies with the rest of the body. We 

 must therefore assume that the common cloacal cavity of the Pyro- 

 *<>n«i colony is a new structure which arises later, though its develop- 

 ment has not so far been described (p. 403). 



The primary enteric rudiment in the Ascidiozooids gives rise first 

 directly only to a pharyngeal cavity which functions as a respiratory 



