PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 539 



In the apical region of a branch, as shown in the upper part of text-fig. 2, A, the 

 ostia are closer together than on the sides of the branch, and the tubes that open to 

 the exterior are much shorter than elsewhere ; but the tubes are of the same calibre as 

 in the other parts of the branch. The substance of the coenoecium is more delicate 

 and the lining of the tubes thinner and less brown in colour than in other parts, and 

 the tubes have no spines. These circumstances lead to the conclusion that the growth 

 in the length of a branch takes place at the apex. 



Text-fio. 2. — A , a diagrammatic longitudinal sectioB of a branch of C. agglutinans. a, a short peripheral 

 tube at the ajjex of the branch ; b, b, b, b, a jiart of the inner tubular system showing a complete open 

 circuit ; c, coenoecial substance, a soft albuminoid with shelly particles included ; d, d', parts of tubular 

 passages cut obliquely : they are not blind ends ; I, lip or spine set at the opening of a peripheral tube ; 

 s, s', thin concavo-convex septa set across the tubes. Approximately x IJ. For further explanation 

 see the text, page 538. 



B, a stout branch of C. agglutinans with the ostia and their spines or lips indicated over a jjortion of 

 the surface The tube-openings are shown in black : the oval and crescentic areas represent the broken 

 bases of spines, a, a', spines situated on the side of the tube towards the apex of the branch ; b, b', 

 spines situated towards the basal side of the tube ; c, c', spines more or less lateral in position ; 

 s, s', spines with no tubes alongside them. Approximately x 1^. See text, p. 541. 



In the case of species of Idiothecia and Orthoecus it may be surmised from a study 

 of the characters of the tubes of the coenoecium that young zooids, on separating from 

 the stalk of the parent zooids, migrate over the surface, and settle down, in the case 

 of species of Idiothecia* at the apex of the branch, and in the case of species of 

 Orthoecus at the margin of the colony-mass. In C [Demiothecia) insequatus, 



* Except in G. indicus (Schepotiepf, 09), about which we know too little to be able to speculate upon the rnode 

 of growth of the colony. 



TRANS. ROY. SCO. EDIN., VOL. XLIX. PART III. (NO. 7). 71 



