1891.] Mechanical Origin of Structure in Pelecypods. 15 
employed in crawling. In later stages the animal adopts the habit 
of byssal fixation, and then of free swimming. The foot becomes 
highly reduced, and in the adult is probably a functionless organ. 
In shells which are permanently attached by calcareous fixation 
the foot is highly reduced or absent, as in Chama, Spondylus, 
and Ostrea (Fig. 4). In Ostrea the shell becomes permanently 
attached at the close of the free-swimming veliger stage; therefore 
the foot is unnecessary before fixation and useless afterwards, and 
it has almost entirely disappeared from even embryonic stages of 
` growth. In Anomia glabra the foot is active and well developed 
in nepionic stages ; but later, as the animal becomes permanently 
attached, it is reduced, and in the adult is highly atrophied. 
Ordinarily there are two posterior retractor muscles of the 
foot in Pelecypods, one situated on either side. In adult Pecten 
either the left retractor alone exists, or both retractors are wanting 
(the left doubtless always exists in the young). In studies of 
young Pecten irradians I found that the animal always crawled 
while lying on the right side, with the foot extended through the 
notch in the lower valve and pressed against the surface of sup- 
port. It is evident that while crawling in this position the left 
retractor is in the plane of traction, and it is retained; on the 
other hand, the right retractor would not be in the plane of trac- 
tion, and it has disappeared through disuse and atrophy.’ A sim- 
ilar disappearance of the right retractors of the foot is seen in 
Anomia glabra, and is explained on similar bases of argument. 
The action of the foot in its effect on the form of the shell in 
some Cases presents interesting mechanical features. In dimyarian 
Pelecypods which crawl freely the foot protrudes from an area on 
the free border of the valves, nearly opposite to and comparatively 
far removed from the hinge line. In such cases, by a slight 
gaping of the valves, a considerable opening is made, through 
which the foot extends without (ordinarily) the aid of any special 
notch. On the other hand, in monomyarian Pelecypods, where 
the revolved position of the axes brings the foot close up to the 
hinge line, a special notch is required for the extrusion of the foot, 
as the valves would have to gape very widely to permit of its 
T Both tors probably exist in t h stage of Pecten and allies. 
F 
