Development of the Marine Sponges. 393 



racteristic club-shaped spicules present (fig. 20), which, having 

 their free points in contact with the dermal (ectodermal) mem- 

 brane, had forced ' the latter outwards so as to destroy the 

 even continuity of its surface and thus render it monticular 

 (fig. 17, cc) ,• so that, of these two instances, we have in one 

 (fig. 16) the retraction of the dermal cilia and the development 

 of the first spicules before the latter begin to push outward the 

 dermal membrane, and in the other (fig. 17) an increase of the 

 triradiates with the addition of the club-shaped surface-spicules 

 after the latter have begun to push out the dermal membrane 

 — respectively analogous to similar stages of development in 

 the embryo of Halichondria simulans, described under the 

 " Fourth Period." 



Here it should be added that, on crushing the latter specimen 

 (fig. 17) for more minute examination, not only did the soft 

 contents come out in the state of sarcode charged with cells of 

 uniform size like the last, viz. about 2-G000ths inch in dia-, 

 meter (fig. 17, dd), but many of these cells, which were iso- 

 lated, were also monociliated (fig. 17, f), and others appeared 

 in an aggregated, pavimental form as fragments of the anipul- 

 laceous sacs (fig. 17, a), while after a short time many of the 

 isolated cells began to put forth pseudopodia and creep about 

 the slide after the manner of Amoeba? (fig. 17, </). But in no 

 part was there any appearance of the root-cells, any more than 

 in the former specimen. Thus it was evident that, at this 

 period, the spongozoa and ampullaceous sacs, although still 

 very soft and delicate in structure, had been developed. 



Afterwards it was easy, as before mentioned, to find speci- 

 mens of the fully developedyoung Grantia compressa] and, from 

 observing that the triradiate spicules were confined to the body 

 and the club-shaped ones to the surface in these specimens, 

 together with the triradiates appearing first and by themselves 

 in the previous stage mentioned, it was not unreasonable to 

 infer that the triradiates of the body were the spicules first 

 developed. The fully developed young Grantia compressa 

 (rigs. 18 & 19) now measured in the body (fig. 18, a) 7 by 

 4-1800ths of an inch in its greatest diameters, while that of 

 the ciliated active embryo (fig. 13) at the time of leaving the 

 parent measured 5^ by 4-1800ths of an inch. Still we have 

 to add to the former the distance of the ends of the club-shaped 

 surface-spicules from the surface of the body (fig. 18, c), as 

 the dermal membrane must have covered them when they 

 Avere first developed, and have only left them naked when the 

 sarcodal contents withdrew themselves inwards to form the 

 purse-like body. If, then, we add this distance, it will raise the 

 measurement of our fully developed or embryonal Grantia 



