32 
Proceedings of the Poyal Society of Edinburgh. [Sess. 
is not so trivial as may seem at first glance, for dark colour in the chitin 
of Hy droids is frequently associated with robustness of minute structure. 
It is so in this case, for while the hydrothecae of Sertularella glacialis are 
sturdy and perfect in form, where they have not been broken off by force, 
those of Sertularella plectilis have practically all collapsed, in spite of 
the fact that they do not show any other signs of injury. Hickson and 
Fig. 10. — Part of branch of Sertularella 
glacialis , with hydrothecte. x 30. 
Fig. 11. — Part of branch of Sertularella 
plectilis, with hydrothecse. x 50. 
Gravely also noticed that in their specimens the walls of the hydro thecae 
were “ extremely thin.” 
The shapes of the hydrothecae differ in the two species : in Sertularella 
glacialis a hydrotheca is narrow at the base and widens upwards; in Sertula- 
rella plectilis the reverse is the case, for the aperture is of less diameter 
than is the basal portion. The proportions of the hydrothecae also differ : 
in Sertularella glacialis the hydrotheca is more robust, for the length is 
less relatively to the breadth than in Sertularella 'plectilis ; moreover, 
while in the former only half or very slightly more than half is free, in 
the latter, according to Hickson and Gravely, two-thirds is free, although 
