330 THE AMERICAN NATURALIST. |. [Vor. XXXIX. 
movements of the snout during feeding. The head is evidently 
swept slowly from side to side while the animal dacks gradually 
along the surface on which it is feeding, removing the food as it 
goes from a band whose width is equal to the diameter of its own 
mouth and whose length, disposed in a series of nearly parallel 
but connected arcs, is proportionate to the length of the feeding 
time. Removal of the tissue is effected by a licking motion of 
the odontophore or “ tongue ” with its superimposed radula but 
I have not been able to detect any independent motion of the 
radula although I have made an especial search for a * chain-saw 
movement" such as Huxley described in certain Heteropods, 
and Geddes claims to have seen “in live limpets turned over on 
their backs," and although there are in Acmea fragilis muscles 
which seem adapted for the production of such a motion. I 
have also been unable to gather any evidence as to the way in 
which the individual teeth of the radula are used.  Pilsbry has 
suggested that in Rhipidoglossa the rounded form of the. 
subradular cartilage probably determines a greater activity of 
the median part of the radula and that this in turn accounts for 
the greater specialization of the teeth in that region. 
Respiration is in general aquatic and the mantle no less than 
the gill serves as a respiratory organ, as is demonstrated by the 
disposition of the blood vessels and the course of the flow. 
Water passes in and out beneath the slightly raised shell and its 
character may very possibly be tested as among some Pelecypods 
by the minute tentacles which fringe the mantle. The shell is 
usually thus raised so long as the limpet is undisturbed though 
the slightest disturbance causes it to be tightly pressed against 
the supporting rock. 
Atmospheric air may, however, under certain circumstances 
be respired. When the water in the aquarium has become 
unduly warm or otherwise deprived of a part of its air, it is a 
common sight to see the limpets about its edges with their bodies 
half out of the water, the shell raised to the highest point, and 
the head stretched forward so that the nuchal cavity may be as 
widely as possible opened to the air. Such a position seems 
explicable only on the theory that the animals are seeking to 
supplement the imperfect gaseous exchange permitted by the 
water by a true aérial respiration. . | 
