448 Mr. H. J. Carter on 



tion ; while the external layer of the smooth part becomes 

 poriferous and supported by an additional but slighter skeletal 

 framework, more or less composed of single spicules inter- 

 crossing each other, which support in their interstices the 

 pore-structure ; thus the smooth portion of the tubulation 

 (fig. 1, a, h), which is imperforate for about two thirds of its 

 course, becomes poriferous in the last third of the branches 

 (fig. 1, /). Following now the structure of the latter, that is 

 the wall of the poriferous portion, we find that it consists 

 from without inwards of first a layer of small epithelial cells, 

 rendered cribriform by a great number of pores (fig. 5, bbbb) 

 and supported on a framework of slender intercrossing spicules 

 (fig. 5, cc) ; second, a skeletal layer, which consists of the 

 longitudinally fibro-reticulate spicular layer, now transformed 

 into a quadrangular fibro-reticulate one (fig. 5,aaaa); and 

 lastly the sarco-fibrous layer (fig. 4, a), which has assumed 

 the structure above mentioned, in which the clathrous holes, 

 which are infundibular (fig. 4, Z* Z> 5), open by circular, con- 

 tracted, sphinctral apertures under the pores (fig. 4, cccc), so 

 that, by placing the object between the eye and the light, the 

 pore-structure of the surface may be seen through the infun- 

 dibular spaces (fig. 4, dddd), showing that whatever passes 

 throuo'h the pores must fall directly, without the interposition 

 of any canals, into the tubular cavity of the branch, thus 

 affording an undoubted instance of the " mode of circulation 

 in the iSpongida " to which 1 have alluded in the ' Annals ' 

 of 1885, viz. that the whole of the water and its contents 

 which enters through the pores passes directly into the interior 

 of the sponge before the nutritive particles of it are deflected 

 towards their destination in the spongozoa of the ampulla- 

 ceous sacs or elsewhere ('Annals,' vol. xv. p. 119), for 

 there are no excretory or any other canals here to re- 

 ceive it. Spicules of two kinds, viz. skeletal and flesh- 

 spiculcs : — 1, skeletal spicules of two sizes, the largest, 

 elliptically inflated at one end, followed by a straight fusiform 

 shaft, ending in a smaller inflation of the same kind at the 

 other end, about 55 by l-1800th in., and the lesser one a little 

 thinner, cylindrical, and undulating, but similarly although 

 less inflated at the extremities (fig. 2, a, b) : 2, flesh-spicule, 

 a birotulate, consisting of a thick straight shaft, slightly 

 swollen in the middle, terminated at each end by an umbrella- 

 shaped head (fig. 2, c, and fig. 3, a, b) consisthig of eight or 

 more compressed ribs, each of which radiates from the centre 

 of the summit backwards and outwards to a free point, while 

 the inner or concave surface of the arched rib is united to the 

 shatt by a thin falciform septum ; total length of the birotu- 



