143 



I have described these organs as generating from the surface of the 

 membrane, not as a series of separate and perfect cells, but as long, 

 sacculated projections, the apex of each being crowned by the cyto- 

 blast which originated it, by the projection of the membranous tissue 

 from its circular edge, in a backward direction ; and it is singular 

 that we have in the periostracum of Trigonia Lamarckii, a series of 

 sacculated tissues, which are strikingly analogous in the mode of 

 their development to those which I have described as occurring in 

 Cellepora pumicosa. If the periostracum of Trigonia Lamarckii be 

 detached from the shell by the action of dilute hydrochloric acid, and 

 examined in fluid between glasses, with the outer surface towards the 

 eye, it presents the appearance of a beautiful network of broad yellow 

 fibre, having the interstices filled with a convex membrane, on the 

 centre of which there is a large circular cytoblast, as represented in fig. 

 3, and also in fig. 4, PL xv., as seen with a power of 1020 linear. If 

 the inner surface of the membrane be presented to the eye instead of 

 the outer one, the appearance of the tissue is considerably altered, 

 and under these circumstances the structure resembles a series of short 

 honey-comb cells, having the cytoblasts at their concave terminations. 

 There is no membrane whatever at the bases of these sacculated pro- 

 jections, nor can any indication of one be discerned between that 

 part and the inner surfaces of the cytoblasts, while on the outer con- 

 vex surfaces, the membranes surrounding those organs, although ex- 

 ceedingly pellucid, are yet readily to be distinguished by frequent 

 corrugations of their surfaces. The terminations of the sacs on the 

 outer surfaces are frequently wholly or partially surrounded by a 

 double fine, as if in their advance beyond the plane of the reticulated 

 tissue, they had carried with them, to a certain extent, a marginal 

 fringe, composed of a portion of the thread of the network. No in- 

 dications of vascular tissue were observed on the outer surface. The 

 bases of the sacs, viewed with the inner surface of the periostracum 

 towards the eye, have usually the appearance of being formed of a 

 thin, rounded, marginal edge, which sometimes has much the aspect 

 of vascular tissue, although I do not think, from all the appearances 

 combined, that it is so. But if we obtain a view into the sac, in a 

 diagonal direction, there is frequently the appearance of a vessel 

 crossing its side at right angles to its axis, about half-way between 

 its base and apex ; and from having frequently observed this appear- 

 ance upon the inner surfaces of the sacs, I am much inclined to be- 

 lieve that such a tissue exists in that situation. 



From the close approximation of these sacs to each other, they are 



