2 Mr. II. J. Cai'tcr on three Pachytragous Sponges 



Thus I find that it is only when a portion of Pachymatisma 

 is torn from its natural place of growtli, and becomes free in 

 the sea, that it assumes a spheroidal form. Spreading hori- 

 zontally in its natural habitat, on inclined surfaces, in the 

 most sheltered bowers of the shore-rocks, it rises more or less 

 into obtusely rounded eminences, which give to its sur- 

 face a deeply undulated form. This surface, too, as is well 

 known, is incrusted with a cortical layer or zone of globular 

 crystalloids, which, although thickest on its free side, is never- 

 theless continued all round the sponge, and frequently extends 

 some distance into the mouths of the larger exhalant apertures 

 or oscules ; while the exhalant system of canals is also more 

 or less horizontally developed, and not radiating, in accordance 

 with the mode of growth of the sponge. Hence, when a por- 

 tion becomes detached from its natural habitat and free in the 

 sea, it soon sm-rounds itself entirely with the thick incrusta- 

 tion ; while, its excretory canal-system and general structure 

 continuing as before internally, it can have no radiated ar- 

 rangement of the latter, however much its foiTn may become 

 spheroidal externally. The same applies to the Tethyadte, of 

 which T. cranium is the t}'pe. Thus it may be observed, in 

 my description and illustrations of T. oxahica (Annals, vol. iv. 

 p. 1, 1869), that I found specimens of this species growing in 

 a fixed hemispherical and in a free subspherous form respec- 

 tively. But, as the fixed form has a radiating structure, so, 

 when a portion has been detached from the rock (for it may 

 be assumed always to commence life in a fixed form), it retains 

 this radiated structure in the spheroidal one. Then, as the 

 spheroidal form is accidental in both the free specimens of 

 Pachymatisma Johnstoma and Tethya arahica^ we cannot pro- 

 perly call them " subspherous sponges." This, too, may be 

 the case with the Geodida3 and Tethyadae generally. 



The only true instance of a spheroidal sponge is seen in Tethya 

 lyncurium and the like, where the sponge grows into tliis form 

 on a pedicel, — and here not always, as some of the specimens of 

 this sponge which grow on the rocks of this place demon- 

 strate. At the same time it should be remembered that this is 

 not a Tethya^ if we are to regard T. tranium as ty[3ical of the 

 Tetliyada3 ; and hence Dr. Gray has very properly adopted 

 Nardo's name of " Donatia'''' for this genus (J. c. p. 541). 



Still it may be asked how it is that the spheroidal specimens 

 of Tethya lyncurium^ when cast upon the shore, always pre- 

 sent a facet from which the pedicel has been broken off, while 

 no such indication of previous attachment appears on the sub- 

 spherous specimens of Tethya proper, Geodia^ and Pachyma- 

 tisma (see my illustrations, '■ Annals,' I. c). 



