PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 433 



have been referred to Demiothecia, as well as between those which have been placed 

 in Orthoecus. 



According to the subgeneric diagnoses which have previously been given, C. 

 agglutinans should come in Demiothecia, since the cavity of the ccenoecium is con- 

 tinuous throughout (as distinguished from Idiothecia, in which each zooid inhabits a 

 separate tube of its own), while the tubes are not in the main distinct from one 

 another and with separate cavities, as in Orthoecus. But in other respects the 

 ccenoecium of C. agglutinans shows a greater resemblance to Idiothecia, to which we 

 are inclined to refer it, in spite of the continuity of the ccenoecial cavity. 



The ccenoecium of the known species of Demiothecia consists of a more or less 

 branching system traversed by a continuous cavity, which is of such a kind that a 

 transverse section of a branch shows a central cavity surrounded by a layer of the 

 ccenoecial substance, irregularly thickened in various places. A transverse section of 

 the ccencecium of C. agglutinans does not show this simple arrangement ; but it 

 exhibits a massive ccencecial substance, crammed with foreign inclusions, and traversed 

 here and there by the relatively small tubes inhabited by the zooids. The arrangement 

 of these tubes is shown diagrammatically in text-fig. 2, A, on p. 415. In its massive 

 character and in the disposition of the outer parts of the tubes this has considerable 

 resemblance to the coencecium of C. (Idiothecia) nigrescens, as figured by RIDEWOOD 

 (07 1 , pi. iv. fig. 10). It differs from it in the fact that the tubes which lead to the 

 external ostia do not end blindly at their lower ends, but are connected together by a 

 complicated system of branching tubes which occupy the axial part of the branch. 

 But the fact must be emphasised that the arrangement of the outer parts of the tubes 

 agrees substantially with that of the entire tubes of C. nigrescens, and that no species 

 of Demiothecia yet described possesses a system of regularly arranged long tubes lead- 

 ing from the interior to the ostia. The ostia of Demiothecia are little more than 

 perforations of the common ccencecial wall, so that the central cavity may be described 

 as opening directly to the exterior by means of the ostia. We think, therefore, that 

 there is much to be said for the view that C. agglutinans may be regarded as an 

 Idiothecia in which the inner parts of the tubes of the zooids are connected by a set 

 of tubes, branching in the axial region of the ccenoecium and placing all the cavities 

 in communication with one another. 



A further argument in favour of this view may be obtained from a consideration 

 of the mode of growth which appears to be indicated by a study of the coencecium 

 (cf. p. 415). We have given reasons for believing that the growth of the branches 

 is to a large extent apical, as appears to be the case in C. nigrescens and C. levinseni, 

 both of which belong to Idiothecia. This may be inferred by the regular arrangement 

 of the outer parts of the tubes, in all parts of the colony, as well . as by the fact that 

 the apical tubes are shorter than the others, and apparently younger. 



The occasional occurrence of septa across the tubes of C. agglutinans (p. 414) may 

 indicate some approach to the more typical species of Idiothecia in which each tube 



(BOY. soc. EDIN. TRANS., VOL. XLIX., 557.) 



