PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 409 



The difficulty of finding reliable specific characters in the structure of the zooid is 

 increased by the fact that the material has as a rule not been preserved with any 

 special refinements of technique. The study would be greatly facilitated by being 

 able to make use of specimens which had been preserved in a fully extended condition. 

 It would then be comparatively easy to ascertain the number of the arms or plumes 

 with certainty. Under other circumstances this point, for instance, cannot always be 

 made out without the possibility of mistake. Even in a well-prepared series of 

 sections it is sometimes excessively difficult to count the arms, which may be cut in 

 planes most unfavourable for study. The method of dissection is capable of giving 

 valuable evidence ; but here, too, mistakes are not out of the question. There is 

 reason to believe that in some species of the genus the number of arms is variable 

 (cf. RIDEWOOD, 07 1 ). It must also be remembered that in some cases the number of 

 the arms has been given as the result of the study of sections of a very small number 

 of individuals. The comparison of series of sections of the same species might 

 suggest differences which are really due to the degree of maturity or of contraction, 

 or to the planes in which the sections are cut. Some caution is thus necessary 

 both in making statements and in accepting those which have been made by other 

 observers. 



It can hardly be doubted, however, that the number of pairs of arms does provide 

 a character which is of value systematically. Other characters which can be specially 

 relied on are probably : the number of buds, whether few or many, which are borne 

 at any one time by the budding region of the stalk ; the presence of one or three 

 nerve-tracts in the stalk, as pointed out by HARMER (05) and ANDERSSON (07) ; the 

 presence or absence of deep pigmentation of the skin ; the shape of the operculum 

 or postoral lamella ; the size of the free ova and the structure of the embryos ; and 

 the mode of development of the arms in the buds. 



Using some of these characters, the single species dredged by the Scotia may be 

 thus characterised : 



Cephalodiscus agglutinans, n. sp. Colony massive, branching, somewhat resembling 

 that of C. nigrescens in the size of its branches, but the material of the ccenoecium 

 includes large quantities of shells of Foraminifera, small fragments of shells of Mollusca 

 and spines of Echinoids, and rounded particles of slate. The pieces of colony vary in 

 size up to 100 or 115 mm. in greatest length, and 45 or 55 mm. in greatest breadth. 

 Cavity of the cceuoecium in the form of a tubular labyrinth continuous throughout 

 (except for a few isolated septa), much branched in the interior, but the superficial 

 parts of the tubes are radially set, and open at a fairly definite angle (about 80) to 

 the surface. Diameter of the peripheral tubes 1 mm. Ostia elliptical, size 1'2 by 

 8 mm., each with a single thick lip or spine* projecting about 3 mm. beyond the 

 general surface. No peristomial tubes. Mean distance from the middle of an ostium 

 to the middle of the ostium nearest to it, 3 or 4 mm. ; at the free extremity of the 



* The spines are broken off in the greater part of the material examined. 

 (ROY. soc. EDIN. TRASS., VOL. XLIX, 533.) 



