11 



do not develop in aquaria, tlioiigh thoy may live for days 

 afterwards. Ilatschek has observed (o) that the Vivipo- 

 rous larvae of a species studied by him (T. navalis?) are 

 almost alv/ays present in only -i few stages, and that trans- 

 itional stages are but seldom found. It seems probable 

 that the free-living larvae of Xylotrya and T. norve";ica 

 attain one of these stages within a short time, and that 

 the unusual conditions in aquaria prevent their advance 

 beyond it. 



The mode of life, and rate of development beyond the 

 early stage attained so rapidly in aquaria, have not been 

 determined. Y/hat becomes of the larvae that hatch from 

 the eggs, how and where they live, it is difficult to sur- 

 mise. Though the developed larvae are settling on wooden 

 structures constantly, I have not taken them and interme- 

 diate stages in the tow-net, and v/here they develop I do 

 not know. However, the rate of growth of the marine La- 

 mollibranchs is slow, and I think the larvae of "Ship- 

 worms" that attach must be at least a month old, it may be 

 more. At this time their development is quite advanced 

 and their organization complex. (See th.-- description of the 

 organisation of the larva) 



