Compensatory Regulation. 37 



bears a direct relation to the direction of the coil of the tube. 

 Caullery and Mesnil ('96) have shown that the operculum of 

 Spirorbis and its relatives is always located on the side next to the 

 concave surface of the coil, i. e., on the right hand side in dextral 

 tubes and on the left-hand side in sinistral ones. Whether or not 

 any such relation can be made out in other members of Group VI 

 it is not possible to say. In Ditrupa there is a slight curvature of 

 a definite form in the tube but the relation of the body within the 

 tube has not been made out definitely enough to determine the 

 significance of the constancy of position of the operculum. In 

 Pomatoceros triquetroides there is a definitely fixed left-handed- 

 ness though the tube is irregularly coiled, and in Vermilia multi- 

 varicosa there is an almost equal distribution between the two 

 sides, the tube being again irregularly coiled. 



A discussion of the factors controlling the determination of the 

 position of the opercula must be left until the ontogenetic and 

 regeneratory development of these organs have been studied. 



4. Exceptional Degrees of Development in General and One 

 Case of a Supernumerary Operculum. 



The two opercula in Groups IV and V in the vast majority of 

 cases consist of a fairly typical functional and a fairly typical 

 rudimentary operculum. There are, however, exceptions to this 

 statement, as has already been indicated above. The main 

 exceptions are due to the partial further development of what 

 probably would otherwise correspond with the rudimentary oper- 

 culum, either with or without the loss of the functional operculum. 

 There thus arise either one functional operculum and one partly 

 developed functional or a missing operculum and one partly 

 developed one. In other individuals both opercula were found to 

 be rudimentary or one rudimentary and one partly developed 

 one were present. The discussion of these cases must be reserved 

 until we come to the experimental part of the paper. 



One case of a supernumerary operculum was found and this is 

 interesting in connection with the problem of the regulation of the 

 opercular development. The accompanying figure (Fig. 9) gives 

 the relations of the opercula. On the left-hand side there is a 

 rudimentary operculum of the typical form in the usual position. 

 On the right-hand side in a corresponding position is a typical 



