Mr. li. J. Cartel' on the Ultimate Structure of Spougilla. 25 



being raised with the pellicular covering of the Spongilla ge- 

 nerally, until it presents a considerable angle of elevation at the 

 circumference ; while the parenchyma, either by contraction 

 within, or by forcing outwards bundles of its large smooth spi- 

 cules, here and there separates itself from the pellicular cover- 

 ing, and thus both the investing membrane and its cavity are 

 formed (PI. I. fig. 1 b, b, b). The investing membrane, now 

 supported in its position by these bundles of spicules {d, d, d), 

 and kept on the stretch by the small spiuiferous spicules which 

 are scattered through its substance, presents two objects well 

 worthy of descrii)tion, viz. the peculiar cell to which I have 

 before alluded, and a number of apertures (/, /). 



The cells of the investing membrane are characterized by 

 their uniformly granular composition and coloui'less appearance. 

 They are nucleated, possess the contracting vesicle singly or in 

 plurality, and are spread over the membrane in such numbers, 

 that it seems to be almost entirely composed of them ; while 

 they are of such extreme thinness, and drawn out into such long 

 digitated forms, that they present a foliated arrangement not 

 unlike a compressed layer of multifidous leaves, ever moving 

 and changing their shapes (figs. 6 </ & 7). This is, as I have 

 before stated, the same kind of cell as that which forms the 

 cortical layer of the seed-like body at a very early period ; and, 

 as will be seen hereafter, is further characterized by not enclosing 

 any carmine when the other cells become charged with it. 



The apertures, on the other hand, are circular or elliptical 

 holes in the investing membrane among (in ?) these cells. They 

 seldom exceed y^o^^ ^^ ^^ ^'^^^^^ ^^ diameter, have a clean, thiu 

 margin, which in one part presents a slight tubercular enlarge- 

 ment, and are generally surrounded by some minute colourless 

 granules ; while they have the remarkable property of closing or 

 dilating like the pupil of the eye, but generally with extreme 

 tardiness instead of the velocity observed in the latter (fig. 6 a). 

 The tubercle looks very like the nucleus of a sponge-cell, and, 

 when the apertui'e is contracted, the granules may be seen to 

 be enclosed in a circumscribed form, which, together with the 

 presence of one or more contracting vesicles, gives the whole 

 very much the appearance of one of the sponge-cells peculiar to 

 the investing membrane. Through the apertures the particles 

 of food and other substances suspended in the surrounding water 

 are admitted into the cavity of the investing membrane, pre- 

 paratory to passing into the parenchyma, in the manner which 

 will be presently mentioned*. 



* These are the apertures to which I alluded in the postscript men- 

 tioned. Mr. Bowerbank also discovered them about the same time in 



