Development of the Marine Sponges. 335 



The fourth period occupied just a week, viz. from the 4th 

 to the 10th of August inclusively ; and the embryo was ex- 

 amined in the way above mentioned twice a day, viz. morning 

 and evening, during this time. 



On the 4th and 5th it altered very little in appearance from 

 what has been above stated, beyond becoming a little wider and 

 shorter (PI. XXII. fig. 28). 



On the morning of the 6th the now unciliated ectodermal 

 layer seemed to have descended from the body of the embryo, 

 and, in a homogeneous and transparent form, to have spread 

 out on each side in a denticulated manner, much like that of 

 an Amoeba (fig. 29, «a), by which the body, now erect and 

 conical (fig. 29, Z*), but still opaque, white, and smooth, became 

 more firmly fixed to the pebble. The papillary eminence 

 also appeared to have somewhat subsided into a depression or 

 excavation (fig. 29, c). 



But in the evening of the 6th the embryo, still continuing 

 the same in other respects, had lost its smooth .even surface, 

 and now presented a monticular or polygonal one of a more 

 or less globular form (fig. 30, «), with the vent more pronounced 

 (fig. 30, Z»). It was evident, from what I had seen before in 

 the development of the young Bpongilla from the seed-like 

 body to which I have alluded, that the spicules in the interior 

 of the embryo of Halichondria simulans were being arranged 

 into a skeleton-structure in which their ends, in bundles, forced 

 outwards the dermal membrane of the embryo, and thus gave 

 rise to the irregular monticular surface mentioned. 



During the 7th and 8th days, the projection of the spicules, 

 still under or within the ectodermal membrane, became more 

 pronounced, the body more expanded at the base, and the 

 homogeneous transparent ectodermal expansion more or less 

 withdrawn (fig. 31). 



On the morning of the 9th the ectodermal membrane had 

 become separated from or raised into a kind of film all over 

 the free surface of the embryo (fig. 32, cc), by the projec- 

 tion of bundles of spicules from the opaque white body of 

 the latter (fig. 32, b b), thus causing the opaque portion 

 (fig. 32, a a) to be surrounded throughout above the base by 

 a hollow interval (fig. 32, dd), bounded by the ectodermal 

 layer on one side and the opaque body of the young sponge, 

 now fidly formed, on the other. To the ectodermal layer and 

 this cavity I had, in my paper " On the Ultimate Structm-c of 

 Spongilla^'' given the names respectively of " investing mem- 

 brane" and "its cavity" ('Annals,' 1857, vol. xx. p. 24, pi. i. 

 fig. 1). The latter forms the " intcrmarginal cavities " of Dr. 

 Bowcrbank — strangely, figured from the same sponge as our 



24* 



