BY R. VON LENDENFELD, PH.D. 331 



the larvse of Sponges compared with those of other Coelenterata 

 are only met with apparently in the Calcispongise. The Cerao- 

 spongiae have precisely the same larvse as some higher Coelen- 

 terata, Eucope for instance. Of course one might consider the 

 larva? of Eucope as consisting of two layers, and Balfour does 

 that, but it is the cpiestion whether this procedure is not somewhat 

 arbitrary. One might consider it also in the following light : 

 Into the cavity of a Blastosphsera cells enter from the Ectoderm, 

 which in time fill this cavity, and in this manner form a coeno- 

 blastem. Now delamination occurs in this coenoblastem and 

 hereby it separates into a Mesoderm, and an Entoderm which 

 surrounds the delamination cavity and (conipai'e also Kowalensky's 

 original plate, fig. 8) consists of cells, which 'are differentiated in 

 another manner as the cells forming the Mesoderm. 



Also the second reason of Balfour's admits of a discussion. 



The Sponges are a very old branch of the Coelenterata and for 

 a long time sessile in the adult stage, probably for a longer time 

 than any other sessile Coelenterata, as we can easily conclude from 

 the early time of the affixing of the larvae and the numerous 

 adaptations which the Sponges have attained in connection with 

 this way of living. 



The sessility again caused the appearance of a skeleton here as 

 in nearly all other cases where animals become sessile, which, 

 whether it be of a calcareous, silicious, or horny nature (with the 

 exception of a few very rudimentary Sponges) increased in size 

 and density, from generation to generation, as most important 

 to the Sponges. But the skeleton is not only a product of the 

 Mesoderm of these organisms, it is even its principal product, and 

 so the Mesoderm must have also increased in importance and size, 

 the more the Sponges ceased to be solitary individuals and the 

 more they commenced to form colonies, until it finally, as we see 

 in many adult Sponge-colonies, overgrows the gastral cavities 

 and depresses them nearly to a rudimentary organ. 



The otherwise rare obliteration of mouth and gastral cavity 

 which is sometimes met with, is caused by changes which have 

 taken place in their function in the Sponges. These changes 



