NOTES ON THE STRUCTURE OF OIKOPLEURA. 41 



show distinctly the arrangement in more than one layer 

 noted above (see fig. 10). The entire ventral surface is 

 now covered with large ectoderm cells, and there is a con- 

 tinuous layer of test all over the body. With the excep- 

 tion of the small dorsal nerve cord (n.s.), the branchial 

 sac or pharynx is the only organ inside the ectoderm. It 

 is roughly of triangular form with a ciliated pad projecting 

 inwards from each of its sides. The two lateral pads are 

 the peripharyngeal bands and the third is a median ventral 

 ridge (the hypopharyngeal) which shortly becomes con- 

 verted into a ciliated groove leading forward to the 

 endostyle. Six or seven sections further on (PL IV. fig. 1) 

 this ventral ciliated area is seen as a shallow groove, two 

 sections in front of that (PL IV. fig. 2) it has become a 

 deeper, narrower groove, and in another couple of sections 

 it forms, along with the peripharyngeal grooves (p.p.) 

 which have now moved down veiitrally and coalesced 

 with it, the sloping lateral edges of the aperture of the 

 endostyle into the branchial sac (PL IV. fig. 3). 



The posterior end of the endostyle is found about section 

 220. PL IV. fig. 1, en., shows its appearance in section 

 223 where it is composed of a mass of glandular cells lying 

 half-way between the ventral surface of the branchial sac 

 and the ectoderm, and having no connection with the 

 pharynx. In section 225 (PL IV. fig. 2) the endostyle is 

 larger and has a central lumen around which the glandular 

 cells are placed ; it has now nearly come into connection 

 with the deep hypopharyngeal groove. In another couple 

 of sections we find this connection established as is shown 

 in section 228 (PL IV. fig. 3) where the endostyle has a 

 considerable cavity opening by a narrow slit into the 

 ventral part of the pharynx. The glandular cells are not 

 equally developed around the whole wall of the endostyle 

 but are arranged in four definite longitudinal tracts. After 



