ON TUB DIPLOOHORDA. 307 



into two horns running l)ack laterally (PI. 20, fi<^. 24) to meet 

 the collar coeloni. Here the mesoI)lastic walls of each cavity 

 meet each other and form a well-marked mesentery^ which is 

 of considerable firmness, and probably has in its centre a thin 

 lamina of chondroid tissue. 



In the mid-dorsal line the posterior wall of the pre-oral coelom 

 borders the subneural sinus (PI. 20, tig. 20, nies.) ; and just 

 where the pre-oral mesoblastic wall slopes aw^ay on either side 

 of the sinus (PI. 21, fig. 42) there are a pair of thickenings, ^j. p., 

 which, traced forwards, show themselves to be the commence- 

 ment of a pair of internal openings. Further forward they 

 become cellular tubes (PI. 20, fig. 2S,p.p.), and after a short 

 course parallel to the body-wall they open into grooves on 

 either side of the nerve-ganglion. A cross-section of the tube 

 shows a single layer of columnar ciliated cells, closely similar 

 to the cross-section of the collar nephridium (PI. 21, fig. 41). 

 The pre-oral coelom, therefore, opens to the exterior by a pair 

 of ciliated pores, identical in position and structure with the 

 proboscis pores of Cephalodiscus (see below). 



The pre-oral coelom is traversed in every direction by fine 

 protoplasmic filaments, with here and there, especially at the 

 unction of the threads, a nucleus (PI. 20, fig. 28, b. c, 1). 



The collar coelom is a spacious cavity separated in front by 

 the mesentery {mes. in the figures) from the pre-oral coelom, 

 and behind by the mesentery {mes'. in figs, 22 and 23) from the 

 trunk cavities. It is produced into each tentacle, and is every- 

 where bounded by typical coelomic epithelium. A dorsal 

 mesentery is present throughout their length, in connection 

 with which is a dorsal vessel. We may specially notice that 

 as the notochords grow forward they push the splanchnic 

 coelomic epithelium before them, and they are thus, like the 

 rest of the hypoblast, surrounded by a layer of mescblast, in 

 this case forming a mesoblastic sheath to the notochordal 

 diverticula (PI. 21, figs. 30, 31, sh.). The mesentery [mes'.) 

 separating the collar coelom from that of the trunk starts just 

 posterior to the tentacles and collar ring, and runs forwards 

 and inwards to meet the gut. Its course is shown in trans- 



