of the Fishery Board for Scotland. 



343 



as wide as its geographical range. The existence of the species can be 

 traced hack from the present day to Tertiary times, shells being found in 

 the Scottish glacial beds, and in the Red and Coralline Crag formations 

 of Suffolk. 



II. Habits of the Clam. 



Unlike its relative, the oyster, the common scallop is in its adult state 

 free, and is not perforce compelled to dwell in one place. It can move 

 about in search of food, but the journeys which it undertakes must be 

 rather limited in range. Most other bivalves move slowly by means of a 

 highly muscular organ called the foot. This organ is capable of protrac- 

 tion and of withdrawal, and the locomotive function is further assisted by 

 the ejection through the siphons of the water contained in the mantle 

 chamber. In Pecten, however, the foot always remains in a comparatively 

 undeveloped state, and in the adult stage is, so far as we know, function- 

 less. The animal has the ' power of flight,' and darts through the water 

 in a zigzag line by the flapping of the two valves of its shell. This 

 flapping is brought about by the alternate contraction and relaxation of 

 the single large and well-developed adductor muscle. The adductor, which 

 is composed of innumerable muscular fibres, stretches from the right to the 

 left valve, and on the application of a nervous stimulus contracts, thus 

 closing the valves. The shell is opened by the simultaneous relaxation of 

 the muscular contraction and the action of the elastic ligament which 

 unites both valves along the hinge line. When the muscle is in a state of 

 contraction the ligament is like a piece of cane bent on itself, and rapidly 

 assumes a straightened position whenever the opposing force of contraction 

 ceases, the result being the separation of the valves and the opening of 

 the shell. By a rapid series of contractions and alternate relaxations the 

 valves perforin a flapping movement, and the animal darts hither and 

 thither. But even this power of rapid movement is limited, and it is 

 doubtful whether some of the more quickly creeping molluscan forms may 

 not at the end of a day's journey show as good a record in the distance 

 attained from the morning's starting point. 



While the adult leads this free existence, in its younger days, like the 

 ordinary mussel, it is moored to the tangle, to shells, or to stones. It is 

 fixed to these neighbouring objects by hair-like processes which arise from 

 a groove on the posterior surface of the small foot. These filiform struc- 

 tures are the hardened excretions or products of a special organ, the byssus 

 gland, which is well developed in the embryonal bivalve, and in some persists 

 throughout adult life. While, therefore, the foot of the adult scallop may 

 be without any important function, yet during its larval and post-larval 

 history it contained an actively secreting organ, analogous to the spinning 

 glands of the spider and the caterpillar. The products of the glandular 

 secretion — the byssus threads — anchor the young animal and enable it to 

 withstand currents and storms, which periodically destroy many shell-fish 

 all along our coasts. On the West Coast great quantities of shell-fish, 

 especially littoral species, are often cast ashore. Burrowing shell-fish, like 

 cockles, spout-fish or razor-fish, Myce, and others, after the sand of the 

 banks in which they are embedded has been loosened, and the banks 

 broken up by the action of the waves and breakers, are thrown on the 

 shore. These periodic storms furnish the inhabitants of some of the 

 Western Isles with a plentiful supply of food or bait, if they care to profit 

 from the riches of the sea. Not unfrequently the molluscs so uncovered 

 are allowed to die and decompose on the beach. 



It is highly probable that the ' extraordinary quantities of fine cockles ' 



