SITUS INVERSUS IN ECHINOIDS 145 



p. 338) it is desirable to experiment on the effect of different 

 amounts of diatom-food upon the development of the larval 

 organs. 



The effects of hunger were observed both by E u n n s t r o m 

 (25, pp. 254-321) and Mac Bride (15, pp. 339-40). The 

 difference between the results of these two observers is remark- 

 able. In every instance of E u n n s t r o m ' s larvae showed 

 extreme degeneration of skeletons, while in MacBride's 

 case the larval arms were almost normal, owing to the well- 

 developed state of the skeletons, but the hydrocoele degenerated 

 and peculiar spines formed. Besides the differences in degree 

 and duration of hunger, the stage at which the larvae were 

 treated, &c., there must be still other complicated factors 

 which caused such different results. For those starved larvae 

 bacterial infection is no doubt another important cause of 

 abnormal development (25, pp. 273-4). Grave (9, p. 36) 

 remarked that among the larvae of Mellita only those 

 well fed developed the echinus-rudiment. 



As to the effect of other chemical and physical environments 

 upon the development of the sea-urchin larvae we have those 

 valuable results obtained by Vernon, Tennent, and 

 others. But we know hardly anything with regard to the 

 changes of coelomic vesicles and hydrocoele treated specially. 



9. Summary and Conclusion. 



1. Under artificial conditions more than 10 per cent, of the 

 larvae of Echinus miliar is exhibited the situs inversus. 



2. So far as I could examine, the internal as well as external 

 structures of such abnormal larvae were mirror-images of 

 those of the normal larva. 



3. The young sea-urchins metamorphosed from such inverse 

 larvae showed no abnormal features externally. 



4. The manner in which such abnormal larvae departed 

 from the normal development seems to be analogous to that 

 in the case of ' compensatory hypertypy ' in the claws of 



Alpheus. 

 6. In an early stage of the normally- developing larva it 



