ON THE DIPLOCHORDA. 347 



though they still show indications of two rows, an anterior and 

 a posterior. 



These sections are all transverse to the epistome, and are 

 not quite parallel to the dorsal surface of the collar. It is pro- 

 bable that in a section in the latter direction the six plumes 

 would diverge from the main branch at about the same level, 

 a conjecture further borne out by the examination of uncut 

 specimens. 



Practically the plumes and their pinnae form a large funnel- 

 shaped framework, with the interior lined by the thinner or 

 atrial epithelium, and the exterior with its twelve grooves 

 running down into the mouth, covered by the glandular and 

 ciliated branchial epithelium. 



In Phoronis a precisely analogous arrangement holds, but 

 in this case the tentacles surround the mouth in the mid- 

 ventral line, and are then produced laterally to form a double 

 row, which, bending backwards, forms an imperfect atrium, in 

 which is situated the anus. In this case there is a similar 

 disposition of branchial and atrial epithelium, but, as the 

 tentacles here surround the mouth, the former lines the 

 branchial cavity, with the mouth at its base, whereas the latter 

 lines the less complete atrium. It is evident that if the outer 

 row of tentacles, meeting mid-ventrally in Phoronis, were 

 removed, and the epistome restored to something like its 

 Actinotrochan proportions, the similarity would be striking. 



One cannot doubt that in Cephalodiscus the cilia of the 

 ventral surface cause currents down the ventral grooves of the 

 plumes, and thence into the mouth, as has already been sug- 

 gested, carrying food particles, entangled in slime, into the 

 alimentary canal, a mode of alimentation analogous to that of 

 Phoronis, and indeed of the Urochorda. 



Organs of the Mesoderm. 



Muscular System. — The muscles are little differentiated, 

 and often consist histologically of simple protoplasmic strands 

 with nuclei. In the pre-oral lobe, or protomere, a great number 

 of these strands cross the coelomic cavity in a definite radiat- 



