REPORT ON CEPHALODISCUS DODECALOPHUS. 13 



do not mix with each other in an inextricable manner. In the same way the beautiful 

 plumes of the Sabellidse and Phoronis have a certain amount of rigidity from their 

 internal skeleton, while their graceful motions and their branchial functions are in no way 

 interfered with. 



No special muscular apparatus can be made out in the pinnules, the covering of the 

 central axis consisting of hy[3odermic cells and granules. Nor would such be necessary 

 in regard to the physiology of the organs, the elasticity of the skeletal axis and its con- 

 nection with that of the main stem being sufficient to keep the parts in a position suitable 

 for their functions without any effort on the part of the animal. The main stem has a series 

 of longitudinal fibres, but their muscularity is doubtful. In any case the motions of the 

 disk would influence that of the entire lophophoral apparatus, especially as its great 

 fan-like muscles arise from the skeleton of the basal apparatus of the arms. 



The bases of the arms are hollow and in communication with the two great cavities 

 (one on each side) of the region (PI. VII. fig. 3, cv) which Mr. Harmer, on good grounds, 

 identifies with the coUar-spaces of Balanoglossus. In section the basal spaces are 

 generally filled with fibres detached from the walls, but in some views definite corpuscles 

 in groups are visible. The latter consist of minute rounded bodies with a central 

 nucleus. From the structure of the parts it will thus be apparent that though probably 

 pervious none of the arms show a clear median channel except at base and apex. The 

 paired cavities connected with the lophophoral apparatus communicate with the exterior 

 by a weU-marked and comparatively large pore on each side in front of the gill-slits. 

 These pores present a radiate arrangement of the hypodermic wall in transverse section 

 and thus are readily recognised ; while in certain longitudinal sections a more or less 

 urceolate aspect is produced. 



The tentacles of Loxosoma are stated by Professor Vogt and others to be devoid of a 

 central chamber, and the central axis of the same organs in Pedicellina^ is only 

 cellular (and translucent). Even in Rhabdopleura careful examination under most 

 favourable circumstances by Professor Lankester gave no indication of a median canal, 

 even in the main stems. He was unable to detect any definite cell-structure in the 

 skeletal tissue, but observed that it had a refringency indicating a certain density, and 

 presented small twisted filaments and particles within its substance at intervals. The 

 relation of the twisted filaments to the fibres described in the main stem of each plume 

 in Cephalodiscus is a subject that requires further investigation, and the same may be 

 said of the "particles" which occurred at intervals — in relation to the nuclei already 

 described. Whether Rhabdopleura shows any indication of the lacunge at the base of 

 the lophophore is a question also requiring determination, though if such had existed it 

 could hardly have escaped, in the living animal, two observers of such experience as Sars 

 and Lankester. 



1 Vid£ Nitsclie, Zeitschr. f. wi»s. Zool., Ed. xx. p. 22, Taf. iii. figs, 1, 2. 



