ON THE DIPLOCHORDA. 395 



(fio-. 47), and in most the anus opens to the exterior, though 

 the aperture is very minute. The intestine seems to be 

 hypoblastic in origin. 



In the mesoblast there are considerable changes. The 

 niesobhist cells have now formed the thin "lining-mem- 

 brane '^ type of endothelium, — in fact, typical coelomic endo- 

 thelium, not to be distinguished from the coelomic endothe- 

 lium of the free Actinotrocha with six pairs of tentacles. 

 The several portions of the coelom have come together and 

 formed typical mesenteries. 



The pre-oi-al ccbIoui or protocoele does not otherwise differ 

 in extent from the stage with one pair of tentacles, except 

 that the two horns have reached back to meet the mesocoeles 

 or collar cavities on either side of the oesophagus, which also 

 grow forward from their former position to effect the junc- 

 tion. At the point of junction are formed a right and left 

 mesentery (fig. 49), but in the middle line dorsally the pro- 

 tocoele and mesocoele do not meet, but leave a hsemoccBle 

 space just under the nerve-ganglion (figs. 49 and 46, sns.). 

 This is the subneural sinus, differing only in size from that 

 of the later Actinotrocha. Other ha)mococle spaces can be 

 observed, such as that below the oesophagus in fig. 49, but 

 the extent of these vascular spaces is difficult to trace, and 

 probably varies to a considerable extent. The mesocoe]es(7nsc.) 

 have extended dorso-laterally round the stomach, to meet 

 dorsally just on the fore-part of this organ (fig. 47), whilst 

 they expand widely in a lateral direction, and dorsally at 

 their posterior part, giving off branches to each tentacle on 

 either side. In fig. 48, a transverse section at the level of 

 the post-oral ring of tentacles, the two mesocoeles may be 

 seen pushing dorsally, their walls forming a pair of con- 

 spicuous mesenteries with the walls of the metacoeles {mtc). 

 On either side of the anus and slightly ventral to it the meso- 

 coeles open to the exterior by a small pore (fig. 47), which is 

 evidently the mesocoelic pore or collar pore. There can be 

 little doubt that this is later metamorphosed into the collar 

 nephridium of the later Actinotrocha, probably by the 



