Compensatory Regulation. 53 



mary functional one. Instead it takes on the characters of the 

 adult Hydroides operculum with two rows of serrations. Beside 

 the inverted cone with serrations around its upper edge there is 

 an additional circlet of pointed and often hooked processes, which 

 constitute the most important character of the Hydroides group 

 as distinguished from the Serpula group. At the same time the 

 broken stump on the left side has started to develop a knob of 

 embryonic tissue which grows only up to the stage represented by 

 the rudimentary operculum of the adult and is in its turn restricted 

 in its further development by some force most likely similar to the 

 one which in the first place restricted the original primary rudi- 

 mentary operculum. There are, therefore, at this time a secondary 

 functional operculum on the right side and a secondary rudi- 

 mentary operculum on the left side. These have the essential 

 characters of the opercula of adult specimens of Hydroides. 

 However, one point of difference is evident in specimens taken at 

 random from the sea. It is found that approximately the same 

 number have the operculum on the left side as on the right 

 though there is uniformly a slight advantage in favor of the 

 right-handed ones (57 per cent right-handed to 43 per cent 

 left-handed in H. dianthus), while all the larvae appeared first as 

 left-handed ones and later by reversal changed to right-handed 

 ones. How does this change occur.? Either we must suppose 

 that the similarity in character of all the larvae was accidental or 

 that the reversal takes place in nature during the life of the 

 individual more than the one time described for the young animal. 

 The first supposition seems improbable because the larvae came 

 from a great many different individuals, and moreover the order of 

 appearance was found to be the same in the two Naples species 

 (H. uncinata and H. pectinata). We are, therefore, forced to 

 assume a further reversal as taking place in nature. This 

 reversal may be a purely physiological one, induced by the 

 normal activities of the animal, like the first reversal already 

 described, or may be induced by some injury to the functional 

 operculum of the character which was found to cause such a 

 reversal in my experiments. (See below, p. 55 jfl) However, 

 unfortunately, there is no experimental evidence to show that 

 physiological reversal takes place in nature after the first time 

 already described. As is mentioned elsewhere (p. 65) in this 

 paper the experiments undertaken to determine whether worms 



