SITUS INVERSUS IN ECHINOIDS 117 



surface, for instance, with the result that a minute spiral 

 structure may be taken as turned in a wrong direction. In 

 my case it will be quite sufficient to state that as the larvae 

 were fairly large objects under the microscope, I used to focus 

 by means of the coarse adjustment while examining them with 

 respect to the symmetry relations. 



(2) It is now impossible to compare our culture with the 

 larvae belonging to the same species which might have been 

 found in plankton near Plymouth in the early summer of the 

 same year (1920). One may suppose that if quite a number 

 of naturally-developed larvae were examined carefully there 

 might also be found some such abnormal forms. I think one 

 may safely say, however, that at least the occurrence of this 

 abnormality in so high a percentage as more than 10 per cent, 

 is really due to artificial conditions. 



(3) In view of the fact that in our cultures such larvae with 

 inverse situs were eight times as numerous as the doubles 

 (157 : 19), I cannot help doubting that the previous workers, 

 who were fortunate enough to discover a few double-hydrocoele 

 specimens from among hundreds of larvae, would have over- 

 looked those inverse forms which might have been more 

 frequent. It is very desirable to know if situs inversus occurs 

 also fairly frequently in other species of sea-urchins when 

 artificially reared. 



(4) As stated above, the five lots of cultures shown in the 

 table were obtained from three different parents. It is highly 

 improbable that such a remarkable case, if inheritable, was 

 found in at least three individuals out of seventy sea-urchins 

 (more than 4 per cent.) which had been sent from Plymouth. 



From all these considerations I am driven to conclude that 

 the occurrence of the abnormaUty is true, and can even be 

 fairly frequent among artificially-reared larvae. 



4. Changes which may possibly have taken place 

 DURING Earlier Stages. 



One of the most remarkable and well-known cases of situs 

 inversus among animals is that of the snails with sinistral 



