SITUS INVBESUS IN ECHINOIDS 139 



development of an echinus-rudiment, no matter on which side 

 of the larva it may lie, the hydrocoele, working together 

 with other ectodermal and mesodermal tissues, can induce 

 this new structure to appear and thus the actinal part of the 

 young sea-urchin be completed. 



7. Probable Mechanism whereby Abnormalities are 



produced. 



From those observed facts above considered the following 

 conditions seem to concern the production of abnormalities 

 of the hydrocoele and its associated structures. 



1. Obliteration of the pore-canal. This seems to be a cause 

 of the arrest of the further development of the water-vascular 

 system and then a quick degeneration of the whole system 

 follows. 



2. Activation of the right anterior coelom of its latent poten- 

 tialities of producing a hydrocoele, to compensate the degenerat- 

 ing left hydrocoele. 



3. Kegeneration of the pore-canal or fusion of the two axial 

 sinuses. Both afford the left hydrocoele a renewed com- 

 munication with the exterior, and the further development and 

 differentiation of the water-vascular system thereby take 

 place. 



4. Development of a right amniotic invagination and the 

 peculiar change of the anterior prolongation of the right 

 posterior coelom. These changes seem to have been evoked 

 by the stimulus of the unusual right hydrocoele. These three 

 elements working together give rise to an echinus-rudiment. 



From these data, if adequately combined, the following 

 changes are quite possible. 



Let us start from a young normal larva, in which hydrocoele, 

 axial sinus, pore-canal, and dorsal pore are all formed on the 

 left side. An amniotic invagination may already be formed 

 on the left side. The right anterior coelom may have a pore- 

 canal. 



Now, the dorsal pore of the left side becomes obliterated, 

 which fact is followed by the arrest of development and further 



pro. 261 J, 



