;U0 ROBERT TRACY JACKSON ON THE 



able in the adult. In such tlie watei- is di'iven out so forcibly b}' the rapid ciliated action 

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

 large dish or preserve jar in whieli the animal was. Fui'ther, several times on turning a 

 scallop over, so that the excurrent actiou took place in the op|)osite direction, almost in- 

 stantly the whole mass of water responded and moved in the i-evei'sed course impelled 

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

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

 more or less drawn-out ropy masses, PI. xxviii, fig. 5; in one instance observed, meas- 

 uring 14 cm. in length. Eyder (60) observed that the fsrces 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 shoAV similar results. 



The adult of 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 retrograde 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 boi-der of the valves; in the eai'ly nepionic stage (PI. 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 constantl}', l)ut 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 young are lost. Some species, as Chlamijs (Pecten) varlus, 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 Avas 

 pinched or shadowed; but the animal was ai^parently 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 ampl}' 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 is pi'cvented at the injin-ed 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 ai'ea. This ingrowth is probably induced by the 

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

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

 mal shell de[)osition takes place, the early 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, are 

 noticeable in a study of shell growth. The oyster is mare sensitive to touch than the 



