58 THE SCALLOP FISHERY 



swimming veliger to the larva which swims by a kicking motion of the 

 foot has been given in chapter III. 



The adult swimming characteristics appear at the beginning of the 

 plicated stage, after the mantle has become specialized. During and 

 just previous the scallop has passed through a stage in the evolution 

 of swimming, which, though closely associated with crawling, is the 

 bond that shows its relationship with other lamellibranchs, such as 

 the clam and quahaug. It is the " pseudo-siphon stage," so named from 

 an organ formed by the edge of the mantle on the posterior side of the 

 animal. In crawling, water is ejected from this opening with sufficient 

 force to throw the animal ahead. It is interesting to note that this 

 movement is the first indication of swimming in the animal, and that 

 it is comparable to similar conditions in the other shellfish, which have 

 fully formed siphons in the adult. The description of the adaptability 

 of the anatomical parts of Pecten tennicostatus (Mighels) for swim- 

 ming, as given by Drew (1), applies equally well to Pecten irradians, 

 and is here quoted : 



Pecten is one of the ablest swimmers among lamellibranchs. The whole 

 structure of the animal is modified for this purpose. The valves have be- 

 come rounded in outline, flattened and comparatively light. The anterior 

 adductor muscle has been lost, and the posterior adductor muscle, which is 

 very powerful, is situated near the middle of the body. The cartilage has 

 become well developed, so the shell may be opened quickly when the muscle 

 relaxes, and the hinge line is straight, so there may be no unnecessary 

 strains in opening and closing the shell. Each gill is attached by one 

 lamella only, so water in the temporary cloacal chamber may be thrown out 

 without injuring the gills, and the gills and margins of the mantle are 

 provided with muscles to withdraw them from the margins of the shell 

 when the shell is closed. Furthermore, the margins of the mantle are pro- 

 vided with infolded ridges and with circular muscles, so it is possible to 

 direct the current of water which issues from the shell in the required direc- 

 tion. 



The only striking difference in the swimming of the young plicated 

 scallop and the adult is the extension of the foot by the former, possibly 

 a characteristic retained from the old method of swimming with the 

 foot. In all the swimming of the young the foot is thrust out to its 

 full extent, and possibly assists the animal through the water, either 

 by its waving motion or by its cilia. 



The following excellent account of the method of swimming of the 

 adult Pecten is given by Jackson (4) : 



It is best to study the swimming in young Pectens some 3 centimeters 

 high, as at that age it is more easily seen than in adults, and does not 

 differ from what may be observed in them. Lying on the bottom, with 

 tentacles extended, the scallop suddenly folds the guard tentacles back so 

 that they lie closely against the outer border of the perpendicular mantle 



