1888 .] 
Mr D. M'Alpine on Bivalve Molluscs. 
189 
gill, on the first trial, 26 minutes elapsed from the time of detacli- 
ment until it had travelled an inch, but this was hardly a fair test of 
its speed, as the three succeeding periods of 8, and minutes 
respectively showed. 
The most favourable vertical rate was 7 minutes to the inch, and 
when upside down 2 minutes to the inch. 
A left inner gill placed in sea-water travelled in a horizontal 
direction at the rate of about 1 inch in 3 J minutes. When merely 
moistened it travelled 1 inch in 6J minutes, whether it was on the 
upper or the under surface of the dish. Small pieces of the inner 
gill progressed at an average rate of about 1 inch in 2J minutes, 
especially if the tendency to turning on the posterior end was 
counteracted by placing glass slides on the plate on each side of the 
portion of the moving gill. This movement lasts at least for 48 
hours. The movements of the detached gill will be due to various 
currents differently directed. On its inner and outer surface the 
currents are outwards, on the opposed faces of its lamella the 
currents are inwards, and on the margin of the gill the direction is 
forward. 
Explanation of Direction of Movement . — The gill is the centre of 
various ciliary currents which will drive it in various directions 
when detached. We are so accustomed to and familiar with one 
fixed direction of the ciliary current, that w^e are hardly prepared 
for its diversity in the mussel. In the nose and in the wdndpipe 
the cilia habitually work outwards, but in the gill of the mussel 
they work backwards and forwards, upwards and downwards, and 
in a sloping direction. Three principal currents may be noted, and 
these are not invariable in their direction. 
1. There is the outer surface current from the attached to the 
free margin, which considered alone would drive the gill forward, 
and from the very contour of the gill the posterior end w^ould be 
less rapidly propelled than the anterior end. 
2. There is the marginal current towards the anterior end, and 
this alone would tend to drive round the posterior end and cause 
rotation on the anterior. But this is to a certain extent counter- 
balanced by the slower motion of the posterior end, as indicated 
above, although the main cause lies in the third ciliary current. 
3. There is the intra-lamellar current towards the inner or attached 
