198 Proceedings of Roycd Society of Edinhurgh. [march 5, 
pended in the water are collected and carried to the month without 
any apparent selection. The labial tentacles may possibly have the 
power of rejecting distasteful matters; but it is difficult to conceive 
how this can be, if the particles, as in the present case, always form 
a continuous cord, which would have to be severed before any part 
could be disengaged.’’ The indigo, however, must have entered 
the mouth somehow, for five or six individuals placed in a small 
vessel strongly coloured with indigo, removed it visibly, and it was 
afterwards found cramming the alimentary canal when opened, and 
was passed out, but little altered in appearance, along with fsecal 
matters. This uniting of the loose particles into a coherent mass is 
a convincing argument against the selective function of the palps, 
and it must not be forgotten that if the animal were in its natural 
position, these ravelled threads of matter would not collect in 
heaps about the mouth (as shown in drawings by these authors), but 
would have dropped or been pushed away as each succeeding mass 
came forward. 
According to Keferstein,* feeding is a passive operation, inde- 
pendent of the will of the animal, which accepts whatever the 
ciliary stream brings to the mouth, and this is the view of the pre- 
ceding observers. But I have followed minute particles of sea-weed, 
even to the very margin of the mouth, and they were not mechani- 
cally taken in, but systematically removed. Hence the animal can 
abstain at least from feeding when it wishes, and consequently can 
open or close its mouth for or against the admission of food. 
Material lying immediately in front of the mouth was likewise not 
drawn in. 
In order to come to some definite conclusion about this food- 
question, I first of all determined in a general way what the mussel 
feeds upon. This is easily done by inserting a small pipette 
through the wide mouth and gullet into the stomach, whence the 
contents can be drawn for examination under the microscope. The 
contents are usually of a pale, dirty white or yellowish colour, con- 
sisting of minute organisms and debris of various kinds. 
Diatoms are always present in great variety and abundance, and 
sometimes the colouring matter is seen partially removed. JSTo 
Desmids were found, although carefully looked for. Any amount 
* Bronn’s Theirreiclis, vol. iii. p. 417. 
