PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 417 



The life of a colony is apparently not indefinite, for some of the pieces of the Scotia 

 collection are " dead," i.e. no zooids at all are to be found in them. The tubes are 

 either empty, or are occupied by a powdery brownish black mass, which, judging from 

 its colour, may possibly consist of the products of decomposition of zooids. Among 

 this debris are numerous specimens of a Tanaid Crustacean, of which no adult males 

 have been found, so that the determination of the species is difficult. The specimens 

 have been submitted to Mr T. V. HOIHISUN, who will doubtless refer to them in his 

 report upon the Tanaidacea obtained on the Scotia Kxpedition. 



The spines represented in text-fig. 3 show the mode of their construction by 1 lie 

 superposition of successive caps of coencecial substance and the inclusion of foreign 

 particles in these caps. Each fragment of shell is, of course, embedded in the particular 

 increment that was being applied at the time that tlie fragment was picked up, and 

 consequently it may cause a distortion of the surface of that particular cap, and the 



TEXT-FIG. 3. Three spines showing the successive caps of comoecial substance and the foreign 



particles included. 



subsequent one, but it does not project into the preceding, more basally placed cap. 

 In text-fig. 3 it might appear in certain places as though the particle were pro- 

 jecting into the cap below it, but this effect is produced by the particle and the 

 optical section of the cap drawn in the figure being in different planes; the particle 

 is more or less behind or in front of the median longitudinal section of the spine. 



The diagrammatic text-fig. 2, A, shows that, except at the apex of a branch, the 

 lip or spine at the edge of the ostium may be situated towards the apex or towards the 

 base of the branch, or more or less laterally, and that it has not the regularly basal 

 position that it has in C. nn/rfxcens (07 l , pi. iv. fig. 10). This irregularity is illustrated 

 further in text-fig. 2, B, which represents the carefully drawn details of a part of the 

 surface of a stout branch. Owing to the fact that most of the spines on the surface 

 of a piece of colony are broken off short, it is necessary to dissect into the substance 

 of the ccenoecium to see the relation of the spine to the ostium. In the portion dis- 

 sected it will be noticed that the spine is situated towards the apical end of the branch 

 at and a', but towards the basal side of the ostium at /< and //, and it is lateral Iv 

 placed at c and t/. The spines broaden out as one dissects into the branch, and can 

 be traced along the tubes for about a centimetre, when they cease to be distinguishable 



(ROY. SOC. EDIN. TRANS., VOL. XLIX., 541.) 



