1888-89.] Mr D. M‘ Alpine on Bivalve Molluscs. 
725 
Continued Observations on the Progression and Rotation 
of Bivalve Molluscs and of detached Ciliated Portions 
of them. By D. M ‘Alpine, Esq. Communicated by Dr 
Sims Woodhead. (With two Plates.) 
Part II. — In Fresh-Water Mussel ( Unio ). 
{Abstract.) 
(Read May 6, 1889.) 
In the fresh- water mussel the general results are much the same 
as in Mytilus. Movement of the animal is as a whole right-handed 
and slightly forward, though this is not invariably the case. 
The movement appears to he brought about by the contraction 
and expansion of the foot, which is wedge-shaped, and larger in 
proportion than in the sea-mussel. When portions are detached, 
however, the same four parts, or pieces of them, exhibit decided 
movement as in the sea-mussel. The palps rotate, the gills and 
mantle-lobes also move, but remarkably slowly, while the ventral 
margin of the foot is pre-eminently active ; it is par excellence the 
highly motile detached portion. Where not otherwise stated, the 
palps, gills, and mantle-lobes are always laid out with their inner 
surface uppermost, and for convenience of diagrammatic representa- 
tion, as well as for clearness of explanation, the palps are always 
supposed to start from the half-round of the clock face. Thus right- 
handed rotation will have the successive positions of quarter to, 
hour, quarter past, ending with original position at half past. 
I. The Labial Palps are somewhat transverse, triangular flaps 
on each side of the mouth, situated between the anterior adductor. 
Each pair of polyps, on either side of the mouth, is united along 
the line of attachment of the body, and this forms a groove between 
them leading to the mouth ; they are fawn-coloured, and finely 
striated on their apposed faces. 
1. Right Labial Palps, together rotate slowly to the left at 
different rates, from one round in 6 hours to one in 21 minutes ; 
there is also slight forward movement. 
2. The Inner Palps rotate in opposite directions, the right 
rotating left-handed, and the left right-handed ; the left inner palp 
VOL. XVI. 25/11/89 3 A 
