194 Proceedings of Royal Society of Edinhurgh. [march 5 , 
lates as a long streak, held together by some viscid elastic sub- 
stance, and is gradually driven towards the anterior end as new 
material is added behind. 
The gills not only serve to convey solid particles inward towards 
the anterior end, hut likewise outward towards the posterior end. 
This is done by means of the currents on the opposed faces of the 
lamellae. 
Dr Sharpey discovered,* on putting moistened charcoal powder 
on the surface of the gills, that the finer part of the powder soon 
disappeared, passing into the interlamellar space. There it was 
rapidly carried backwards and out at the excretory orifice with the 
general current. The coarser particles remained on the outside, and 
were carried towards the free margin, thence forward towards the 
mouth. 
My own observation confirms this, as small pieces of sea-weed are 
driven backward and inward in a sloping direction, as shown by 
the dotted arrows in fig. 1, and then they are readily passed back- 
ward and outward along the surface of the body. 
The meaning of this arrangement is evident. The newly intro- 
duced water bathing the gills passes through their interstices, and 
thus the tissues are completely aerated, and the refuse water is 
passed out, either along the body-surface or the inner surface of the 
mantle-lohe. But there is not that distinction here of the “inhalent” 
and “ exhalent ” currents which some maintain, since the pallial 
chamber is not subdivided into a ventral or branchial, and a dorsal 
or anal chamber. 
It may be mentioned here, however, that I have more than once 
seen, when the valves gaped a little, the free margins of the inner 
gills close together, and these would enclose a space posteriorly, shut 
off from the general cavity. 
That the gills serve a respiratory purpose is pretty certain, but 
that they also directly subserve nutrition is doubtful, since the 
finer and lighter particles best adapted for that purpose pass through 
the lattice-work of the gills and out of the body, while it is only 
the coarser particles which are driven forward. 
2. The function of the laUal palps is purely respiratory. In the 
inner, the current passing towards the outer margin and away from 
* Art. “Cilia,” Todd and Bowman’s Cydop(xdia. 
