2i8 ANNALS OF SCOTTISH NATURAL HISTORY 



55. THUIARIA TENERA (G. O. Sars). 



As T. tenera has never been recorded in Britain, I give a short 

 diagnosis of the species for the guidance of British workers. This 

 is the more necessary, as I feel certain that the species is not a very 

 uncommon one in our waters, and that, in all probability, its general 

 likeness to young colonies of Thuiaria argentea has caused it to be 

 overlooked or erroneously assigned to that species. 



Trophosome. The colonies attain a height of between two and 

 three inches, and are characterised by the delicacy and fineness of 

 their build. The stems are unfascicled and straight, but zigzag 

 almost imperceptibly between the branchlets towards the summit. 

 They are divided into long regular internodes, each with three hydro- 

 theoe, two on one side and one placed intermediate to these on the 

 other side. From beneath the lower of the pair a branch arises, the 

 branches being alternate, usually simple, and lying in one plane. 

 The branches are divided into long internodes which generally bear 

 three pairs of hydrothecae. The hydrothecae are subalternate, but 

 they vary much in the details of shape (see Broch, ' Die Hydroiden 

 der arktischen Meere,' in "Fauna Arctica," 1909, p. 172, fig. 27). 

 They are always slender, flask-shaped below, the upper portion, which 

 juts out from the internode more or less abruptly, tapering to 

 the orifice. Nutting ("American Hydroids," part ii., 1904, p. 70) 

 describes the margin as " sometimes being round and without teeth, 

 and sometimes being curved, with two teeth of regular Sertularian 

 type. In many cases the margin is produced into a thin collapsible 

 tube. Operculum usually composed of one flap attached to abcaul- 

 ine side of margin, but sometimes composed of two flaps." While 

 the hydrothecse in some of the colonies I have examined agree with 

 Nutting's description in having tubular terminations and single-valved 

 opercula, these seem to me to be secondary features apparent where 

 the original margin of the hydrotheca has become obscured by the 

 regeneration of successive new margins, each new margin retaining 

 the stamp of its predecessor less and less clearly. Even in such 

 cases the primary hydrotheca of the series shows two lateral lobes, 

 and indications of two valves in the operculum, the abcauline being 

 much larger and more perfectly formed. 



Several abnormalities occur. In one a branch has been replaced 

 by an elongate oval hydrotheca which, but for its base, is altogether 

 free and projects in line with the existing portion of the branch. In 

 another, there occur abnormally long cylindrical hydrothecse similar 

 to those I have described and figured in Sertularella quadridens 

 (" Proc. Zool. Soc.," 1 9 1 o, p. 8 1 9, fig. 7 9). Again a branch bifurcates 

 into and terminates in two gonangia (fig. 5), while many gonangia 

 on one colony are abnormally attenuated and are marked by deep 

 annular constrictions (fig. 4). 



