REPORT ON CEPHA LODISOUS DODECALOPHUS. 13 



stem somewhat stiffly, the curves being for the most part terminal, and thus they do not 

 mix with each other in an inextricable manner. In the same way the beautiful plumes of 

 the Sabellidse and Phoronis ha,ve 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 hypodermic 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 eflfort 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 ground.s, 

 identifies with the collar-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 probalily 

 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 well-marked and comparatively large pore on each side in -fi-ont 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. 



Tin, tentacles oi Loxosoma are stated by Professor Vogt and others tu be devoid ut a 

 central chamber, and the central axis of the same organs in Pcdicellina ' is only 

 cellular (and translucent). Even in Rhahdopleiira 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 ol)served 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 descrilied in the main stem of each plume 

 in Cephalodiscus is a subject that requires fnrth<-jr investigation, and the sauu' may be 

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

 described. Whether Rhahdojyleura shows any indication of the lacunae at the Uino of 

 the lophophore is a question also requiring determination, though if such had exi.steil it 

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

 .and Lankester. 



' ViiU Nitsche, Zdtschr.f. wiis. Zooi, B.i. nx p. 22, Taf. iii. 6ge. I. 2. 



