ON THE DIPLOCHORDA. o81 



fessor Mcintosh in his monograph upon the species. The 

 doubts which have been thrown upon the claims of this form 

 to specific distinction do not seem to be well founded, as it 

 has well-marked differences which cannot be explained as 

 variations. Apart from those which concern the compara- 

 tive length of the adults, the number of tentacles, and the 

 differences in pigmentation, it is noteworthy that the size of 

 the embryos in Ph or on is australis is greater than that of 

 P. Buskii; so much is this the case that it is easy to dis- 

 tinguish the embryos of each species by their relative size in 

 a mixture of the two. 



In P. Buskii the embryos are found in enormous numbers 

 in the outer coil of the lophophore. They are all enclosed in 

 a thin egg-membrane, which is intact in the latest stage found 

 in the tentacles. Rupture of the egg-membrane would 

 appear to be synchronous with escape of the larvee from the 

 lophophore of the parent. 



Fig. 59 is a transverse section of the lophophore and sur- 

 rounding parts of Phoronis Buskii, seen from above. As 

 is well known, the lophophore is composed of a nearly com- 

 plete ring of tentacles surrounding the mouth. This ring, at 

 first nearly circular in those species in which the young 

 stages are known, is drawn out laterally into an ellipse, and 

 then the extension on each side is rolled up dorsally (to- 

 wards the anus) into a spiral. 



The inner (oral) surface of all the tentacles presents a 

 ciliated epithelium of long cells, which probably cause cur- 

 rents of food and water to pass downwards towards the 

 mouth. The outer surface of the tentacles has a non-ciliated 

 epithelium of cubical cells, which are much flatter than the 

 cells of the inner surface. In Fig. 59 the distribution of the 

 two layers is shown as seen in a section about halfway up 

 the lophophore. 



Thus the inner coil of the lophophore leads down to the 

 mouth, whilst the outer coil eventually opens out opposite its 

 fellow in the median line between mouth and anus. Through 

 the greater extent of the lophophore the apertures of the 



