OiSr DEVELOPMENT ETC. OF PHORONIS. 541 



upper and tlie lower walls of the liood. Thus it will be seen that 

 the septum completely separates the preoral cavity from the collar 

 cavity just behind the ganglion. But, when we come to secticms 

 passing through a more lateral regi(jn to either side of the gan- 

 glion or of the œsophagus, the upper portion of the septum Ijecomes 

 abruptly indistinct. In fig. 54, which shows a sagittal secti<)n 

 through the right-hand side of the oesophagus of a larva of 16 

 tentacles (type A), the septum {tiles'.) near its ventral attachment 

 is indicated by a comparatively thick layer of cells, while the 

 dorsal portion is divided into fine protoplasmic branches, of which 

 some extend to the upper wall of the hood and others stop short 

 of it. As the relations of this septum are somewhat complicated, 

 I will try to make them clear by referring to a series of cross 

 sections (not continuous) through the hood of a larva of type D 

 (figs. 59 a-d) and also to the annexed wood-cut. The latter is a 

 diagrammatic representation of the Aclinotrocha hood and its 

 neighbouring part, as seen from above, i. c, in horizontal projection. 

 The dorsal side is above and the ventral, below\ Nearly in 

 the centre is the nerve ganglion. Below it and concealed from 

 sight, is the mouth, from which the œsophagus leads downwards. 

 The little stellate markings, scattered over the greater part of the 

 figure, are supposed to represent meseuchymatous cells, wdiich, with 

 the branched and reticulate fibres arising from them, pervade 

 the preoral l)ody-cavity, except in a small space immediately in 

 front of, and l)elow, the ganglion. This free space I shall call the 

 230sterior recess of the preoral cavity. The line a b c d e f g h, 

 curved somewhat like the letter M, indicates the position of the 

 preoral septum. The part shown in full line represents that por- 

 tion of the septum which is complete in structure and the part in 

 broken line, that portion of the same which is incomplete. All 



