1909.] Hermaphroditism in Mollusc Crepidula fornicata. 473 



these in a square bracket, thus: — [((?)(<?)<?]. This indicates that younger 

 and smaller individuals settled down on members of the main or primary 

 chain, and formed secondary or side-chains ; occasionally even tertiary chains 

 were found. 



A comparison of the chains, even in Table V, brings out the regularity of 

 the positions in which the different categories of individuals occur. As 

 before, the ? 's are found at the bottom of the chains, the <J 's at the top, and 

 the 's in the middle. Between the more distal ? 's and the § 's are found 

 the ? p.r.'s ; between the more proximal <J 's and the § 's or distal females 

 are found the <$ u.r.'s. 



Hence, in a typical chain a series may be constructed to read from the top 

 to the bottom, thus : — $ , $u.r., § u.s., §, §p.s., ? p.r., ?. 



It is interesting to note this series is just what one would expect to find if 

 $ 's passed successively through the different stages indicated, becoming finally 

 ? 's. It is certain, as will be shown later, that such a change does occur. 



All lengths of chains occur from as many as 12 individuals in a chain down 

 to one. A single individual was regarded as settled if its shell fitted 

 accurately the irregularities of the surface upon which it was found, and if 

 this surface were found to be clean. It is to be remembered that the records 

 do not bring out the facts that in all chains there is typically a decrease in 

 size from proximal to distal individuals. 



In reviewing the records it is seen that a chain of a given length may be 

 formed by various combinations of the five sex-forms (see chains 91, 338, 340, 

 in Table V, p. 476), but the relative positions in the chains of individuals of 

 the different categories are, with rare exceptions, constant. Occasionally it 

 was found that an form occurred between two 's. 



Chain Formation. 



From a study of the records therefore, it would seem that chains are formed 

 as follows : — 



A male settles down on some surface, but before another male creeps on 

 to its back it may pass through the series of changes from J 1 to ? as shown 

 in Table II. Chains of one individual may be said to be at Stage I, chains 

 of two individuals at Stage II, and so on. 



At any time while the single male is changing into a female, another male 

 might creep on to its back, settle down permanently, and form a Stage II 

 chain. The new comer may then change to a female, and thus a Stage II 

 chain might be found in any of the conditions represented in Table III. 

 Similarly, another male might settle down upon a Stage II chain at any 

 condition of the latter, and, changing in turn into a female, would form a 



