Individuals of the smaller species, Pectens irradians, do attach 

 themselves with a byssus and I have no reason to doubt that indi- 

 viduals of this species attach themselves. An individual of 

 Pecten irradians placed in a glass dish of sea water will some- 

 times protrude its foot from the shell, apply it closely to the 

 bottom of the dish and after a short time slowly withdraw it, 

 leaving a rather broad band of slightly yellowish material 

 attached to the glass and connected with the foot at the byssal 

 gland. This is not composed of small tough threads as in the 

 mussels Mytilus and Modiola, but it may be sufficiently tough 

 to support the weight of the animal if, after a few minutes, the 

 dish is carefully turned over. The animals seem never to remain 

 attached for long periods, but after a few hours at most the 

 attachment is dropped at the byssal gland. Whether this is 

 passive or due to a sudden strain caused by forcing strong cur- 

 rents of water from the shell as in swimming has not been 

 observed. 



As in other lamellibranchs the foot is largely composed of 

 crossing muscle fibres that by individual or combined action may 

 press upon blood that may be confined in a rather large blood 

 space in the foot and so cause the elastic foot to be extended. 

 The foot is attached to the shell by a single retractor muscle 

 which runs along the dorsal portion of the foot posteriorly, 

 dorsal to the posterior adductor muscle, to be inserted on the 

 left shell valve at about the point where the adductor muscle is 

 separated into two parts (fig. 10, fm.). This muscle extends 

 along the dorsal border of the foot and is about equally in evi- 

 dence on its right and left sides. Above the opening of the 

 byssal gland, which lies somewhat to the right of the median 

 line on the ventral side, the muscle loses its individuality and 

 becomes merged with the general foot muscles. It leaves the 

 ioot along the median line flattens a little and gradually runs 

 over on the left side to be attached to the left shell valve. Why 

 the right muscle should have degenerated is not clear but the 

 position, attached to the left shell valve, which is uppermost, 

 gives the muscle a straighter pull when the foot is attached to 

 the bottom ty the byssus than would be the case if the right 

 muscle had persisted instead. 



18 



