No. 461.] BIOLOGY OF ACMHA. 331 
Limpets give evidence of the possession of only three special 
senses: sight, touch, and the temperature sense. If they are 
placed in water warmer than that to which they are accustomed, 
they betray their uneasiness by restless wandering about. Vis- 
ual sensations appear to be limited to the perception of light 
and darkness. That light is objectionable only when associated 
with heat seems indicated by the following experiment : a limpet 
with the pebble to which it was affixed was removed from the 
shaded to the sunny part of a tide pool. The water, being 12 
to 15 inches deep, effectually protected it from the heat of the 
sun and it showed no desire to move. When the pebble was 
placed, however, in water only an inch or two deep and so con- 
siderably warmer, the limpet exhibited lively discomfort and 
crawled at once toward the shade. I have assumed that the 
light-perceiving organs are the eyes but have taken no steps to 
prove it. 
The sense of touch is possessed by the entire body surface 
though especially localized in the tentacles, both marginal and 
cephalic, which are richly provided with tactile (Flemming's) cells. 
The gill also, although without such cells, has an exquisite sen- 
sitiveness and from its exploratory movements while the animal 
is traveling, would seem to serve as an additional organ of 
touch. 
Though the sexes in Acmea testudinalis are distinct, the 
shells show no sexual differentiation, whether in size or in form; 
in the ripe animal, however, the sex can usually be determined 
by a difference in the tint in that part of the foot which imme- 
diately underlies the generative gland. The eggs when nearly 
ripe assume a tint much like that known among dry-goods mer- 
chants as “crushed strawberry," and the testis at the same 
period is of a golden brown. On the left side of the body the 
gland immediately overlies the foot and is perceptible through 
its tissue as a patch of dull reddish or of creamy brown bounded 
in the median plane by a sharp line corresponding to the plane 
where it abuts against the green nephridium. 
The breeding season appears to be a long one ; I have taken 
ripe limpets near Boston as early as the thirteenth of April and 
as late as the end of July. In Eastport they were still laying 
