PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 413 



irregular iu their disposition, but the cavities may still be termed tubular in spite of 

 irregularities iu their shape. 



The internal tubular system is continuous throughout the whole branch, and if it 

 were not for probable resistance offered by other zooids one could conceive of a zooid 

 travelling internally from any one part to any other part of the system. In the middle 

 portion or core of a branch the tubes not only exhibit branching, but here and there a 

 complete tubular circuit can be seen, as is shown at b, b, b, b in the diagrammatic text- 

 tig. 2, A. The existence of such continuous circuits is explained by the part of the 

 circuit nearest the base of the branch being due to the bifurcation of a tube during an 

 early stage of the growth of the branch, when this part occupied an apical position ; 

 whereas the part of the circuit farthest from the base of the branch is due to the bury- 

 ing of a superficial groove that connected two young ostia at a time when this part 

 of the branch constituted the apex. There can be no doubt, from a study of the apex 

 of a branch, that growth is apical. The tubes that open to the surface at the apex are 

 much shorter and closer together than in other parts of the branch, although of the 

 same diameter. Evidence of the branching of a tube at the apex is forthcoming, and 

 also evidence of the enclosure of a superficial groove between two young ostia. such as 

 produces the continuous circuits above referred to. 



The ostia show on the surface of a piece of colony as dark areas, due partly to the 

 fact that one is looking into a tubular cavity, and partly to the fact that the actual wall 

 of the tube does not contain the shelly particles that make the softer substance of the 

 ccenrecium .so white in colour. In rare cases in the material studied the zooid occupy- 

 ing a peripheral tube could be seen without cutting into the branch, but in most cases 

 the zooids had all retreated into the middle parts of the tubular system, and had become 

 entangled and intertwined with one another. The ostia proper are slightly funnel-shaped 

 and oval, measuring about 1*2 nine, in long diameter and '8 mm. in short diameter, 

 whereas the tube itself is roughly circular in section, and of about 1 mm. diameter. 



So far as can be seen from the study of the surface in protected parts between two 

 branches that have not broken apart, the tubes do not project beyond the general sur- 

 face, as "peristomial tubes," in the manner of the tubes of C. levinseni (05, pi. ii. fig. 11) 

 and the tubes in certain parts of the colony of C. nigrescens (07, pi. iii. fig. 5), and this 

 conclusion is supported by a study of the short tubes at the free extremity of a branch. 

 The filling up of the intervals between the new tubes seems to take place pari passu 

 with the additions made to the mouths of the tubes themselves. (Incidentally it may 

 here be mentioned that the " habit " or " facies " of a colony of Orthoecus differs from 

 that of a colony of Idwthecia owing to the peristomial tubes of the former being of 

 extraordinary length ; indeed, in C. rarus the whole colony consists of peristomial tubes 

 with only a small amount of softer coenoecial substance binding the blind ends of the 

 tubes together.) 



The distinction between the sub genera Idwthecia and Demiothecia was drawn in 

 1907 (07 *, p. 7) because the isolation of the zooids in separate tubes in the coanoftcium 



(BOY. soc. KDIN. TEAKS., VOL. XLIX., 537.) 



