20 W. G. RIDEWOOD. 



persistence of the branching stolon in Rhabdopleura, so that the polypides remain 

 in organic continuity, constitutes another point of difference, and the frailness of 

 the tubarium and the diminutive size * of the polypides and colony, yet other 

 differences ; but on the whole the effect of recent investigations made upon 

 Rhabdopleura and Cephalodiscus is to justify the action of the earlier zoologists in 

 associating these two genera in the same group Pterobranchia of Lankester, 

 Aspidophora of Allman, Discophora of Hatschek. 



DESCRIPTION OF CEPHALODISCUS NIGRESCENS. 



Cephalodiscus nigrescent Lankester, Proc. Roy. Soc., 1905, pp. 400-402. 



Material. 



THE whole of the material of this species at present known was dredged by 

 the 'Discovery' on January 13th, 1902, in 100 fathoms, off Coulman Island near 

 Victoria Land, in the Antarctic Ocean. The bulk of it was preserved in a 

 5 per cent, solution of formalin, but two pieces were fixed by Perenyi's fluid, 

 followed by alcohol, and two pieces by picric acid solution, followed by alcohol. 



Tubarium. 



The colony of this species is bulky and massive, the tubarium f gelatinous in 

 appearance, nearly transparent, somewhat opalescent, and with a slight yellowish- 

 brown tint. The largest piece in the collection measures roughly 190 mm. by 

 115 mm., and has twelve branches (15, also plate 1 of this Report). A smaller 

 but more massive piece is shown in fig. 2, plate 2. The largest single branch is 

 90 mm. long and 32 mm. across. The branches are roughly cylindrical in shape ; 

 the larger ones are blunt-ended, the smaller ones taper towards their extremities. 



Opening at fairly regular intervals over the surface of the colony are the 

 tubes in which the polypides dwell, and the substance of the tubarium is sufficiently 

 transparent to enable one to trace the tubes inwards for a moderate distance with 

 the unaided eye, and to recognise the polypides within the tubes. The lining of 

 the tube, i.e. the layer which bounds the cavity in which the animal lives, is 

 darker in tint, and of tougher and firmer consistency than the common portion 



* Fowler puts the diameter of the polypide of Rhabdopleura at -123mm., and Schepotieff at -15 

 to '16 mm. 



t The soft secreted material that forms the protective envelope or dwelling of Cephalodiscus has 

 almost invariably been called the " coenoecium," a term which was shown by Lankester as far back as 1884 

 (13, p. 624) to be inappropriate, for the coenoecium of the Polyzoa is the locally thickened cuticle of the hinder 

 part of the polypide's body, to which it is permanently adherent. In describing the structure of Rhabdopleura, 

 Lankester explains (13, p. 635) that it is the caulotheca or stalk-pipe of this animal which is the true homologue 

 of the coenoecium of the ordinary Polyzoon colony. "This equivalence," he continues, "makes it all the more 

 necessary to distinguish the tubular dwelling of Rhabdopleura by some other name, and justifies the special term 

 ' tubarium.' The tubarium has no equivalent in Phylactolaemous and Gymnolaemous Polyzoa." 



