Movements of Detached Portions of Bivalve Molluscs. 145 



An outer and inner palp were laid out at 11 p.m. to test 

 how long they would retain movement, and next morning 

 both were found moving. The morning after both again were 

 found to have moved, and on the evening of the same day 

 the inner palp moved \dsibly, while the outer was sensitive, 

 but not motile. Hence one of the palps, at least, retained 

 its power of movement for 48 hours, but this duration was 

 afterwards completely eclipsed by the palp of the fresh 

 water mussel (Unio), which actually continued to rotate 

 for eight days. 



A comparison may now be profitably instituted between 

 the outer and inner palps as to direction and rate of 

 movement, taking the partial average as a fair one. 



Table I. 



Left, Direction. Average Kate. 



Outer - Left-handed, outward - 7^ mins. per round. 



Inner - Right-handed, inward - 3 ,, „ 



Right. 



Outer - Right-handed, outward - 3 ,, „ 



Inner - Left-handed, inward - 2J „ „ 



The outer rotate outward, the inner rotate inward, and 

 this suggests a difference of function which we shall see 

 actually exists. But it is also suggestive of some difference 

 of structure or relative position, and the latter is found to 

 be the case. 



There is not any important difference in the rate of speed, 

 except with the left outer, and its inherent slowness is 

 borne out by two series of observations. No general 

 conclusion can be drawn from the fact, but it remains that 

 the left outer is fully twice as sluggish as the othei's when 

 detached, and even regularly so, for each quarter round was 

 frequently two minutes, thus completing a round in eight 

 minutes. 



II. — Gills. 



The sills will be named as in the following scheme : — 



T- P, f Inner. 

 ^^^^ \ Outer. 



Gills 



Right 



j Inner. 

 \ Outer. 



