No. 461.] BIOLOGY OF ACMAA. 329 
is the first step in determining whether or not it possesses the 
homing power. Dr.W. H. Dall, however, tells me that he has 
observed on the Pacific coast individuals of Acmea spectrum 
Reese whose shell margins exactly corresponded to the irregu- 
larities of the soft rocks on which they live. Fisher notes that 
both this and other species of Acmzea are often found adhering 
to the shells of Lottia in which they have made shallow depres- 
sions. These observations furnish the strongest presumptive 
evidence that in some species, at all events, the homing power 
is present and render desirable the investigation of the ques- 
tion in Acmea testudinalis under conditions more favorable than 
those afforded by the New England coast. 
The food of limpets appears to be exclusively vegetable. A 
freshly taken specimen always has the digestive tract stuffed 
with a finely divided mass. of unrecognizable material mingled 
with which are unicellular algze and bits of the cortical cells of 
some of the larger ones. I have made no attempt to identify 
these organisms. Intermixed with this material are great num- 
bers of tiny stones which probably, like those found in a bird's 
gizzard, aid in the comminution of the food. This is especially 
necessary in the limpet since for the greater part of their extent 
the walls of the alimentary tract are entirely devoid of muscles. 
These bits of stone are very likely, at least in large measure, 
fragments rasped off by the radula from the rocks together with 
the small algze which cling to them. It has been suggested in 
regard to Patella that the bits of alga cortex are in like fashion 
detached, so to speak, by accident together with minute organ- 
isms both animal and vegetable, which live upon the alga and 
which constitute the real food of the limpet. Touching this 
suggestion I can only say that I have never found in. any part 
of the alimentary tract of Acmaa testudinalis remains which 
could be interpreted as animal in nature and that I have found 
the animal apparently feeding upon large alge which so far as 
one could see, were absolutely free from smaller organisms. 
As to habits of feeding I have never been able to observe the 
act, but twice have, as I believe, interrupted it. In each case 
the seaweed to which the animal was attached bore a mark, due 
to the removal of the cortex, which served as a record of the 
