STRUCTURE OP THE LARVA OP SPONGILLA LACUSTRIS. 373 



changes in connection with the plasmodial aggregations 

 of cells. 



The megascleres are produced in the cells with vesicular 

 nuclei, and arrange themselves in various ways at the meta- 

 morphosis. Many of them place themselves almost vertically 

 and raise up the dermal membrane, giving the upper surface 

 of the young sponge an irregular conformation of hills and 

 valleys, as it were. Below the dermal membrane is the sub- 

 dermal cavity, lined by cells with granular nuclei above and 

 below, while in the centre of the little sponge are the large 

 spaces of the exhalant system opening by the osculum. 

 Along the surface, on the outer aspect, are the inhalant ostia 

 opening into the subdermal cavity. The osculum, which 

 occupies a central position, is at first on a level with the sur- 

 face, but soon becomes situated at the tip of an erect oscular 

 tube. All the cavities in the interior are lined by flat epi- 

 thelium cells with granular nuclei, and other cells of this class 

 are also seen in the spaces between the chambers accompanying 

 the cells with vesicular nuclei. The microscleres are secreted 

 by cells with granular nuclei. 



Now that a brief historical account and a summary of the 

 results of the work embodied in this paper have been given, the 

 problem to be solved in the more detailed account which is to 

 follow can be stated. 



It will be necessary to show that there are several types of 

 larvse which diflFer considerably from one another as regards 

 the structure of the inner mass, and that owing to these dif- 

 ferences the metamorphosis of Spongilla may take place in 

 more than one way^ — that is, the metamorphosis depends on 

 whether the larva at the time of fixation has developed in the 

 direction represented by types B and C, or in that represented 

 by type D. 



It will be shown that in the first case there will result two 

 slightly different methods of metamorphosis, the differences 

 between which will depend on the age of the larva. If the 

 larva fixes in the condition of type B, there will be small cells 

 in the interior not yet developed into chambers. If, on the 



