SALPIDAE — FORMS WITH COVERING FOLDS. 



441 



bridge, first elongates. Tin- tral>ecula, which is retained between 

 the two diverticula of the developing respiratory cavity (Figs. 220, d, 

 aiul 222, cl) that run upward, represents the rudiment of the gill (k) 



which, when the diverticula unite over it, becomes detached from the 

 dorsal wall of the respiratory cavity.* 



Fig. 222. — Median section through a later ontogenetic stage of Salpa pinnata (after 

 Salbnsky). <•/. atrial diverticulum of the enteric rudiment ; d, enteric rudiment 

 (rudiment of the respiratory cavity); ec, ectoderm; f, ciliated pit ; h, rudiment of 

 heart : ms, mesenchyme; it, ganglion ; pc, pericardium; />/. tissue of the placenta. 



The part of the respiratory cavity which is to he regarded as t he 

 cloacal cavity is not, consequently, according to Salensky, derived 

 from an independent rudiment, but arises through the formation of 

 diverticula from the rudiment of the pharyngeal cavity. Todaro 



:; '[The recent observations of Brooks, Heider and Korotneff are so abso- 

 lutely antagonistic to those of Sai.knskv. that we must now regard the above 

 account of the origin of the pharyngeal and atrial cavities and of the gill as 

 inaccurate. Eeidsr and Korotneff are fairly in agreement, and together 

 ilie\ differ in some important points from Brooks. All three are, however, 

 agreed that the atrium develops first, and is probably ectodermal in origin. 

 The pharynx, on the other hand, appears as a cleft in the embryonic mass 

 which becomes lined with entodermal blastomeres. The gill arises from the 

 septum between these two cavities by the breaking down of the lateral parts 

 of this septum, the clefts thus formed being the gill-slits. Salhnsky's view 

 that the gill and atrium oi Salpa arc not homologous with the similarly named 

 structures in the other Tunicates may be regarded as disproveh. — Ed.] 



