1909.] Hermaphroditism in Mollusc Crepidula fornicata. 



469 



a. 



i 



Fig. 1. — On the left ; Dorsal View of a single Shell : a, anterior ; p, posterior. On the right ; 

 a Postero-dextral View of a Chain attached to an Oyster-shell (SH). (-J nat. size.) 



A, shell of proximal individual ; B, shell of second individual, and so on. The symbols 

 adjacent to the shells denote the secondary sexual characters of the inhabitants (see text). 



whole, a chain is seen to form a spiral of about half a turn, bending over to 

 the right. 



On close examination of the chains it is found that when an individual settles down 

 upon another it places the right antero-lateral border of its shell close to or touching that 

 of the individual upon which it settles ; since, however, the individual shells are roughly 

 semi-ellipsoidal in shape, with the longitudinal axis a little concave to the right, and have 

 the left side a little more convex dorso-ventrally than the right, it follows that the chain 

 must form a right-handed spiral. 



In a typical chain the twisting is accentuated by the gradual decrease observable in 

 the size of the shells : the shell of the bottom or " proximal " individual being the largest, 

 and that of the top or "distal" individual the smallest. The "proximal'' individual 

 always fixes the chain to some surface, as the shell of a dead or living animal, a pebble, or 

 a piece of broken pot or glass. In America, chains of two or three individuals are found 

 on the ventral surface of the King-crab (2, p. 10). 



The following facts indicate that when a chain is once formed, none of the 

 individuals separate : — , 



(1) The accurate fitting of each shell into the crevices and irregularities of the surface 



or shell upon which it occurs ; hence, only short periods of separation of the 

 individuals could be possible ; no such periods have, however, been observed. 



(2) In soft rock the proximal individuals wear a deep impression of the edge of the 



shell by the lateral movement executed in the search for food. In these cases the 

 surface to which the middle of the foot is attached is not worn down, so that the 

 animal becomes fixed on a boss of rock, which thus fits loosely into the aperture 

 of the shell. 



(3) Animals detached from a surface are apparently incapable of refixing themselves 



after a certain age, which I have not determined ; for, immediately an animal is 

 detached the sucker-like foot becomes "cupped" by a strong contraction of its 

 muscle fibres ; subsequent relaxation of the fibres does not seem to be possible. 



(4) Prof. Conklin states that old individuals sometimes become permanently fixed by a 



calcareous secretion of the foot (2, p. 11). 



YOL. LXXXI. — B. 2 M 



