Development of the Calcispongia3. 43 



ciliated (posterior) half becomes converted into the skeleton- 

 forming layer; whilst the anterior* ciliated section draws 

 back into the interior of the larval body, to form the entoderm. 

 That the half of the body consisting of what Hackel calls 

 flagellated cells actually draws back into the interior is con- 

 vincingly shown by comparing with each other the four larval 

 stages represented in figs. 5 to 8. It will be seen that in each 

 stage it projects less and less, while the skeleton-forming layer, 

 on the contrary, becomes proportionately larger. In order to 

 obtain a notion of the mode in which the ciliated half retracts 

 itself, we must examine larvge which have adhered rather 

 early, i. e. before the formation of the skeleton. In these we 

 can see that, while the hinder half has altered very little, the 

 anterior ciliated section becomes invaginated in the interior 

 of the body (fig. 9), by which means, of course, an aperture 

 (aperture of invagination, fig. 9, o) is produced at the upper 

 pole. The ciliated hemisphere consequently forms a sac-like 

 body, which appears to be surrounded by the skeleton-forming 

 layer. It appears from the next following stages that the 

 aperture of invagination just mentioned does not pass directly 

 into the definitive osculum, but becomes entirely effaced. 

 Hence, in its further development, the young adherent sponge 

 appears as a perfectly closed body, in which two principal consti- 

 tuents may be clearly distinguished (fig. 10). Externally there 

 is the skeleton-forming layer, in which several rod-like spicules 

 are enclosed ; in the interior, on the contrary, there is a closed 

 body, which represents the entoderm. The walls of the latter 

 appear so thick that for a time one is unable to detect any 

 cavity ; this comes later into view, when the double-layered 

 wall becomes formed into a vesicle. In the sponge three days 

 old, represented in fig. 11, I could already observe an internal 

 cavity (fig. 12, c), but it was still very narrow and small. 

 It was only in a larger Sycon, six days old, that a consider- 

 able cavity was to be seen ; it shimmered through the body- 

 walls even in the living animal (fig. 13). When this same 

 animal was treated with acetic acid, the two vesicularly inflated 

 layers t (fig- 14, a, J), as well as the internal gastro-vascular 

 cavity (fig. 14, c), could be most clearly distinguished. In 

 this stage, the latest that I have seen, no buccal aperture was 

 yet formed ; on the other hand, three-pointed spicules were 

 already present. 



* As regards the designations " before " and " behind " I agree, upon 

 developmental historical grounds, with Lieberkukn and Schmidt, but not 

 with Hackel. 



t It is to be remarked that I could detect no cilia on the entodermal 

 cells of this stage. 



