340 ROBERT TRACY JACKSON ON THE 



able ill the adult. In such the water is driven out so forcibly by the rapid ciliated action 

 of the gills that a rapid gyrating current was imparted to the whole mass of water in a 

 large dish oi- ])reserve jar in which the animal was. Further, several times on turning a 

 scalloi) over, so that the excurrent action took place in the opposite direction, almost in- 

 stantly the whole mass of water responded and moved in the i-eversed course impelled 

 by the changed direction of the current. The faeces of Pecten may be seen escaping in 

 pellets, the form of which in section is that of a clover-leaf, and they are produced in 

 more or less drawii-ont ropy masses, PI. xxvin, fig. 5; in one instance observed, meas- 

 uring 14cm. in length. E3'der (60) observed that the feces of the oyster have a distinct 

 almost horse-shoe form, due to the shape of the intestinal tract and in insects the form 

 of the faeces is often a specific characteristic. It is possible that a study of this feature 

 in molluscs might show similar results. 



The adult o^ Pecten irradians is a free swimmer. No indications of a byssal fixation 

 were seen at this age, and the foot is a highly atrophied organ. I have never seen the 

 foot extended beyond the margin of the valves in the adult, although it is so active in the 

 young, and it may then fairly be considered a I'ctrograde and probably functionless organ. 

 There is, therefore, a series of periods of habits in the life of Pecten. In the prodisso- 

 conch stage of Pecten (discussion later, section x), it is either a veliger swimmer, or 

 crawls on the ventral border of the valves; in the early nepionic stage (PL xxviii, figs. 

 1, 9, 10) it is an active crawler, lying on the right valve; in late nepionic and nealogic 

 stages, it is attached by a byssus more or less constantly, but also has the power of us- 

 ing the foot; in the adult, it is a free swimmer and the habits of crawling and byssal fixa- 

 tion seen in the J^oung are lost. Some species, as Ghlamys (Pecten) varius, are said to 

 retain the byssal habit of fixation throughout life. 



Frequently Pectens show an extensive loss or injury of the mantle. One adult speci- 

 men observed had the left lobe on the ventral side entirely wanting, and the correspond- 

 ing lobe on the right side was destitute of the eyes, tentacles and pigment. The 

 damaged lobe was bereft of sensation, the creature showing no response when it was 

 pinched or shadowed; but the animal was apparently uninjured otherwise. Such injury 

 is probably due to the attacks of predatory animals, as when lying with its valves wide 

 open it is an easy prey. Some indications pointed toward disease, and the loss of the 

 mantle may be due to that cause. That scallops sutfer a severe loss or injury to the man- 

 tle and recover, is amply proven by frequent specimens of mended shells. The margin 

 of the shell is not injured by the predatory attacks (if such they are), but further shell 

 growth 1$ prevented at the injured point by the loss of the mantle. When one mantle 

 border is injured, the other repairs the damage which would be caused by local want of 

 shell growth, by curving the shell deposition of the uninjured side rapidly inwards, thus 

 obviating the deficiency of the injured area. This ingrowth is probably induced by the 

 lack of resisting support on the part of the damaged border. Adults which have been 

 thus damaged in youth show that in time the injured mantle boi'der is renewed, and nor- 

 mal shell deposition takes place, the earl}^ damaged period being indicated by hollow- 

 like, uneven depressions in the shell surface. In the oyster I have never seen injuries 

 to the mantle in living animals, and no evidence of serious injuries, later repaired, ai"e 

 noticeable in a study of shell gi'owth. The oyster is more sensitive to touch than the 



