PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 543 



than the majority, and are nearly white when dry ; these are grains of slate more 

 decomposed than the others. 



While the slate-grains are all rounded and water-worn, the calcareous inclusions 

 are remarkably free from signs of attrition. Delicate shells of Foraminifera, minute 

 bivalve Molluscs, extremely thin, and with the two valves still in their natural relation, 

 pieces of Echinid spines, and portions of Polyzoan colonies occur in a remarkably fresh 

 and undamaged state, and can be picked out clean from the soft albuminoid material 

 of the coenoecium. Among the other inclusions one can recognise pieces of fairly large 

 Lamellibranch shells, fragments of Gastropod shells, and pieces of Serpulid tubes. 

 These broken pieces of shell show sharp edges in the great majority of cases, but a few 

 are rounded. The fragments are, many of them, but little larger than the shells of the 

 Foraminifera, but most are from "5 to 1 mm. in longest diameter. The largest pieces 

 picked out were 8, 10, or 12 mm. long. 



The included particles are all clean and free from mud, and from their even 

 disposition in the coenoecium one is tempted to conclude that they have not drifted 

 by accident against newly secreted coenoecial material which has not yet "set" or 

 hardened, but that the particles are definitely selected by the zooids and built into the 

 wall of their colonial residence, somewhat in the manner in which the arenaceous 

 Foraminifera form their shells, and the larvae of the Caddis-flies their tubes ; and 

 similar instances might also be quoted from among the tubicolous Polychset Worms 

 and the Rotifers. 



The inclusion of foreign particles is not always uniform in the same piece of colony. 

 It frequently happens that one side of a branch is whiter than the other, and a longi- 

 tudinal section shows that the whiter half has more calcareous particles embedded than 

 the other, and fewer tubes. 



The pieces of colony brought home by the Scotia are too fragmentary to enable 

 us to draw any conclusions as to the total dimensions of a colony of C agglutinans. 

 None of the pieces show at their basal ends any signs of an area of attachment to a 

 solid substratum, and while each of the larger pieces, such as are figured in PL L, 

 may be a separate colony, it is just possible that they may be portions of a large 

 branching colony. This is rather suggested by all the zooids found being males, and 

 by the fact that all the buds are in about the same stage of development — namely, very 

 young buds, up to buds with four pairs of arms : only two buds were found having as 

 many as five pairs of arms. There is, however, no evidence of the existence of stouter 

 branches than those shown in PI. 1., such as would be capable of sustaining the great 

 weight of the specimens in question, and, further, it must not be overlooked that the 

 shelly fragments in the coenoecium add to the weight of the branches without adding 

 to their strength, and this may impose a limit to the size of an individual colony. 



There is yet. however, a further possibility that the whole colony may consist of a 

 broad, plate-like part closely adherent to some other object, and that from this tliere 

 may stand up a forest of pieces such as are figured in P]. I. The trawl in passing over 



