Compensatory Regulation. 73 



operculum were symmetrically arranged with respect to the 

 median line. 



However, the ontogeny of Hydroides shows an asymmetry 

 from the very first appearance of the opercular modification. 

 Furthermore, the fact that this earliest development always occurs 

 on the left side indicates some correlation between the character 

 of the tube and the position of the organ. In Hydroides, how- 

 ever, there is an irregularity in the coiling of the tube from, the 

 very start, so that we get no evidence here of such a relation. 



An examination of several Serpulids brings out the following 

 relation between the adult position of the functional operculum 

 and the character of the coils of the tube. A tabulation of the 

 result is given below: 



Tahle IX. 



Funct. Operculum I Tube 



Spirorbis Always right Dextral coil. 



Pileolaria Always left Sinistral coil. 



Ditrupa Always left Definite curve. Relation 



to animal not made out. 



Pomatoceros Always left Irregular. 



Vermilia Right or left Irregular. 



Hydroides Right or left Irregular. 



Serpula Right or left Irregular. 



In the case of the definitely coiled forms Spirorbis and Pileo- 

 laria there is a very definite relation between the direction of the 

 coil and the position of the operculum, the operculum being 

 always on the inner side of the opening, /. -?., the side next to the 

 concave side of the coil.^ In Ditrupa this may also be true though 

 the curve of the tube is slight and the relation between body posi- 

 tion and the curve was not made out. The supposition of such 

 a relation between opercular position and the curve of the tube 

 is weakened by the fact that the tube lies freely on the sea bottom 

 and, therefore, may lie on either side as far as known. In Poma- 



^See Caullery et Mesnil ('96). 



