x 
No. 461.] BIOLOGY OF ACMA. 134 
is outward on that side and inward on the left so that the posi- 
tion which I have described would seem on the whole better 
fitted than the reverse one for fertilization. On the other hand, 
however, it should be said that when the animals were killed 
twenty-four hours later, the ova of the female were unfertilized 
and no spermatozoa were observed in her nuchal cavity or neph- 
ridium. It should also be said that in two instances unfertilized 
eggs have been laid in my aquaria, but as the same thing some- 
times occurs with moths when pairing is prevented, no deduction 
can be drawn from this fact. 
The eggs, at least in the aquaria, are laid about nightfall and 
the trochosphere is developed in the course of the next day. 
I have made no attempt to watch the development. Growth 
must be rapid, as one finds at Nahant in September and early 
October large numbers obviously of the season's young, which 
are four or five mm. in length. Sexual maturity is probably 
acquired after the first winter, as I have taken ripe limpets in 
April which were under a cm. in length. 
The only enemy of the limpet of which I have had any 
experience is the purple, Purpura lapillus. 1 have found occa- 
sional shells bored by this mollusc and in one instance was able 
to surprise it in the act. 
