590 
BULLETIN OF THE BUREAU OF FISHERIES 
of foreign material from the shell. Certainly at times the foot is rather active for 
an organ the only function of which is the rare spinning of a byssus. In the very 
young it is, of course, used for locomotion (crawling). 
ADDUCTOR MUSCLE 
The adductor muscle (morphologically the posterior adductor) of the scallop 
(see figs. 3 and 11) is of special interest. The great size of this muscle renders econom- 
ically feasible the practice followed in this country of utilizing it only, of all the tissue 
of the scallop. It is correlated with the unusual lameliibranch habit of swimming. 
The adductor of a very large scallop (89 millimeters long), taken in January, was 
found to weigh 20 grams after draining. 
The adductor muscle is composed of two very unequal parts. The larger and 
clearer is the motor muscle which functions to snap the valves together and provides 
the motive power for swimming. In the scallops it is composed of striated fibers. 
The smaller, milky white portion, sometimes termed “ligament” by scallopers, which 
lies posterior to the larger and is composed of unstriated fibers, functions to hold the 
shell closed or in any partially closed position. It exhibits what has been known as 
the “catch mechanism ” and, therefore, has been termed “catch muscle.” If an object 
be thrust between the open valves, these close sharply upon it and hold. If then the 
object be pulled from between them, the valves temporarily remain as they were. 
Pressed closer together they remain in the new position a time but resist opening. 
Sometimes too forceful opening tears the catch muscle in two. These phenomena have 
been observed by me many times and are well known. To all appearances and in 
effect it is as if the muscle were not pulling the valves together but instead rigidly 
retaining them as by a catch, or better a ratchet, which does not interfere with shell 
closing, but against quick opening pressure is unyielding, unless “thrown out” by the 
proper nervous stimulus. 
Such phenomena have attracted much attention. A very interesting account is 
that of Uexkiill (1912), who reported that certain nerves inhibited the catch mech- 
anism and others brought it into play, but that if the nerves were cut when the 
mechanism was in operation, stimulation of the nerve endings could not be made to 
throw out the catch. The smooth muscle remained at the length it had when the 
nerve was cut. Important earlier investigators were Pavlov (1885), Marceau (1909), 
and Parnas (1910). The latter failed to find evidence, as he believed, of increased 
metabolism with increased strain or exhaustion after prolonged strain. This, together 
with the remarkable ratchetlike functioning, made a good case lor the view that this 
portion of the adductor might be considered as a passive mechanism under nervous 
control, not active muscle. This is most interestingly discussed by Bayliss (1918). 
Increased information and renewed consideration of the evidence have led recent 
investigators to doubt the validity of the catch mechanism hypothesis. Ritchie 
(1928) reviews the published data and some unpublished work in which he shared. 
From the data on metabolism, relaxation time, tension, response to stimulation, etc., 
he comes to the conclusion that, while there are several doubtful points, there is 
nothing known which is incompatible with the view that catch muscle is merely very 
slow muscle of great tension (but not especially great in Pecten whose muscles instead 
are relatively fast). See also Boylan (1928), Waele (1927), and Hopkins (1930). 
Although as the expression of an acceptable hypothesis “catch mechanism” and 
“catch muscle” may be doomed to be discarded, merely as a descriptive name “catch 
muscle” is effective and useful and may be retained. 
