442 TUNICATA. 



(No. 113), on the contrary, states that, in tialpa, the cloacal cavity is 

 derived independently from the ectoderm, two ectodermal invagina- 

 tions that develop one after the other taking part in its formation. 

 An ectodermal growth first develops on the dorsal side, behind the 

 cerebral vesicle, and this then becomes hollow and forms a vesicle 

 which is the rudiment of the primary cloacal vesicle. This vesicle 

 becomes applied laterally to the epithelium of the respiratory cavity 

 (pharyngeal cavity), but remains separated from the latter medianly 

 by a mass of mesenchyme. The median part becomes the future gill y 

 and in the lateral part, at a later stage, the two large gill-clefts break 

 through. Only after the gill is completely developed does a second 

 ectodermal invagination appear and bring about the communication 

 of the cloacal vesicle with the exterior (cf. the development of the 

 cloacal cavity in S. democratic^, p. 426).* 



According to SALENSKY, the development of the gills varies considerably in 

 the different species of the Salpidde. In 8. africana and S.fusiformis the gill 

 develops through a median fusion of two horizontal folds which rise from 

 the lateral walls of the respiratory cavity (Fig. 221, fc).t The figure of S. 

 africana, in which this is depicted, serves at the same time to illustrate some 

 other peculiarities occurring in this species. We see the upright crest (c) of 

 the covering fold (p. 435) in cross-section, and may observe that the connection 

 of the embryo with the placenta is here brought about in a manner different 

 from that in S. pinnata, a large cavity ( p) here being intercalated between the 

 embryonic rudiment and the roof of the placenta. The origin and significance 

 of this cavity are, however, still somewhat obscure. 



According to SALENSKY, the rudiment of the pharyngeal cavity is not closed 

 completely, but, at a certain stage, shows a dorsal aperture through which a 

 considerable number of mesenchyme-cells wander into the respiratory cavity 

 [probably entodermal blastomeres]. These partly fill the latter and become 

 applied to the wall of the intestine from within ; later, however, they disin- 

 tegrate and are absorbed (Fig. 220, z). 



The development of the actual enteric canal (as an outgrowth of 

 the pharyngeal cavity), that of the endostyle, and of the oral and 

 atrial apertures, seem to take place here in the same way as in 

 S. democratica-mucro'nata (p. 430). 



The rudiment of the nervous system (Fig. 218, n), which arises, as. 

 we have seen (p. 439), as a trilobed cell-strand (Fig. 219 A, n, n'), now 



* [TODARO'S account, in its most essential features, is in accord with that of 

 BROOKS. ED.] 



t [KoROTNEFF (No. XXIa.) supports SALENSKY in his account of the origin 

 of the gill in S. africana-maxima. ED.] 



