1888.] 
Mr D. M‘Alpine on Bivalve Molluscs. 
193 
Eeferring generally to the molluscan foot, Professor Lankester* 
writes — “ The most permanent and distinctive molluscan organ is the 
foot. It may be compared, and is probably genetically identical, with 
the muscular ventral surface of the Planarians and with the suckers 
of Trematoda, hut is more extensively developed than are those 
corresponding structures;” and it is interesting to find that the 
movement of the “ genetically identical ” foot when detached is 
similar to that of the Planarians. Their movement is due to cilia, 
and the description of it in the land Planarians by Professor Semper f 
would apply equally to the detached foot of Mytilus. He says — 
“ They move by means of fine microscopically small hairs the cilia 
or flagella which are attached to the skin, and which by their 
peculiar motions can carry the animal forward when it is surrounded 
by a sufficient quantity of trickling water or of mucilage. On a 
perfectly dry surface, therefore, they cannot creep about for any 
length of time ; the rapidly drying skin would soon yield up all the 
moisture which the cilia on the under side require for their motions.” 
Darwin J also says — “ Hone of these [land] species have the quick 
and vivacious movements of the marine species ; they progress by a 
regular wave-like movement of the foot, like that of a gasteropod, 
using the anterior extremity, which is raised from the ground, as a 
feeler.” Leaving out of account the rapidity of movement, the 
looping of the detached foot of Mytilus, its forward movement, and 
its capability of turning round, are all strikingly suggestive when 
compared with a Planarian. 
Bearings of the observed Movements and the Directions of the 
Ciliary Currents on the uses of the Parts. — The functions with 
which we will particularly concern ourselves are those of respiration 
and nutrition. 
1. Gills. — The gills, inner and outer, create currents from the 
posterior towards the anterior end, along their free margin, as well 
as from the attached to the outer edge. The result is, not only 
that a constantly renewed stream of water bathes the gills, and thus 
serves respiratory purposes, but solid matter is likewise sifted from 
the water and carried along the free margin. This often accumu- 
* Ency. Brit, 9th ed., art. “Mollusca.” 
+ Animal Life, p. 186. 
I Ann. Mag. Nat Hist., vol. xiv., 1844. 
