STUDIES OF THE OYSTER DRILL 
99 
Urosalpinx cinerea feeds on almost all bivalves. The author has observed it feed- 
ing on the following animals : Oysters, clams, mussels, Crepidula, small crabs, barnacles, 
and even on its own kind, while other investigators report the drill as perforating 
scallops (Belding, 1910) and chitons (Arey and Crozier, 1919). Contrary to Colton’s 
(1910) results with Sycotypus and Fulgur, the drill will feed on the meat of oysters, 
clams, fish, crabs, etc. 
The drill’s method of attacking an oyster is as follows: After coming upon the 
oyster the drill selects a valve, the choice of which depends upon many factors, moves 
over it, chooses a place, and attaches itself, adhering to the substratum by means of 
the posterior part of the foot. Its proboscis is then thrust out, the odontophore comes 
into play and the rasplike radula begins to drill. After the shell has been perforated 
the proboscis is inserted and the animal feeds. 
The first question that naturally arises is this : Does the drill limit itself as to the 
size of the oyster attacked? Field and laboratory observations support the conclusion 
that it preferably feeds on the smaller and thinner shelled oysters, but that oysters of 
all sizes are subject to its attack. More than one drill may attack an oyster at the 
same time — an observation corroborated by Belding (1910) for scallops. But even 
if one drill succeeds in piercing the oyster shell before the other, the unsuccessful drill 
continues its rasping, a fact shown by several shells which had as many as four dis- 
tinct perforations and by direct observations. 
As in nearly all other marine mollusks the drill possesses an osphradium — an 
organ intimately connected with the breathing organs, being generally placed near 
their base. It consists of a patch of modified epithelium connected by its own nerve 
with one of the visceral ganglia (Cooke, 1895) and, as was shown by Copeland (1918) 
for Alectrion obsoleta and Busycon canaliculatum, is the organ of smell. Through it 
the drill is undoubtedly attracted to the food. 
The selection of the valve to be drilled depends on several factors. The first, 
purely mechanical, is closely related to the age and size of the oyster. For instance, 
oysters at the spat stage or less than 1 year old are so attached to the substratum that 
only the upper or right valve can be attacked. Although this is not so true of larger 
oysters there are other limitations. Oysters lying flat on the bottom have only one 
valve exposed which on natural rock is usually the right, and on planted beds may be 
either. The second important factor in the choice of valve is one which is not at all 
understood. Observation made by others as well as the author (Pope, 1910, 1911; 
Nelson, 1923) show that the drill preferably chooses the thinnest shelled animals — 
a reaction that would undoubtedly bring about the selection of the right shell since 
that is usually the thinner unless some other response prevented this choice. The 
third response, which is perhaps the most important of all, is the negative geotropism 
of the animal. The significance of this is apparent from observations made in the 
laboratory at Hampton Roads and Beaufort. The essentials were these: Individual 
oysters of different sizes were placed in tanks so that almost the whole of the two shells 
were accessible to the drill. The oysters were then arranged so that different parts of 
the shell would be uppermost. Drills were introduced and in almost every case the 
hole was made on the shell lying uppermost. These results contradict the observa- 
tions which showed that the thinner shell is the one chosen. The author believes 
that this contradiction can be explained on the assumption that the effect of gravity 
overcomes the response to thin shell. 
In the field all these factors play an important part, and depending on the indi- 
vidual bed the results will be different. Two illustrations will suffice. A newly 
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