336 ROBERT TRACY JACKSON' ON THE 



jjlace at one definite area. Tliese genera have mantle borders separate around tlie en- 

 tire margin of the shell; yet the outflow is not in any direetion as might be supposed, 

 but is limited to one direction. 



The gills of Peeten of the age of PI. xxvin, fig. 2, consist of four sets of filaments all 

 eqaally developed, but thei-e is none of the in-cgularity of development of the separate 

 gill lainelhe desci'ihed in the oyster. The iiiner ])air of gill filaments are i-etiected in- 

 wards and the outer pair are reflected outwards as in the adult, PI. xxiv, fig. 12. The 

 reflected borders are about half the total length of the direct borders whereas in the adult 

 the reflected bordei-s are proportionately longer. Thiscondition in the young Avas doubt- 

 less pi-eceded by a condition in Avhich the filaments were simple, straight processes Avith- 

 out reflected borders as described in young Ostrea and Anomia. The gills are ciliated 

 but have not yet the ciliated intei-filamental knobs or processes existing in the adult. 

 There are no septa joining the direct and reflected portiou of the filaments, character- 

 istic of the adult, observed at this early stage. 



When a Peeten of the age of PI. xxviii, fig. 2, had been kept in confinement for a few 

 days it became attached by a byssus and from my observations it is probable that, up to 

 this period, the young lead a free, crawling existence without any fixation. "With the in- 

 introduction of the byssal period, as it may be called, the young Peeten undergoes a 

 marked change in habits. Previously an active crawler, it now becomes most persistent in 

 its byssal fixation, and leads a sedentary life. The animal lies on the right valve; the bys- 

 sus is extended through the byssal notch as a number of delicate fasciated thi-eads flat- 

 tened into a disc at the point of contact with the object of fixation. The specimen if 

 kept in a beaker of water frequently attached itself high up on the perpendicular sides of 

 the glass to which position it had previously crawled. It occasionally broke loose volun- 

 tarily, or was intentionally detached and in a short time, conunoidy a few minutes, made 

 a new attachment, showing that fixation is a feature of this condition. It was in the 

 specimen of this early stage, when attached by a byssus, that the pseudosiphonal protru- 

 sion of the mantle was observed. AVhen the animal is attached, the tentacles are more 

 or less fully extended, the foot may be protruded or withdrawn, and feeding is active as 

 indicated by faecal discharges in the excuri-ent water. 



Pectens considerably older than those already discussed, measuring 1-3 cm. in height, 

 are insti'uctive. At this age the byssal habit of fixation still exists; in fact, specimens 

 measuring slightly over 3 cm. have been found still attached, but I have never observed 

 such a condition in adults. Pectens 1-3 cm. high, though retaining the byssal hul)it, 

 ai'e not as constantly attached and are more active, than the younger stage just consid- 

 ered. Two hundred or more individuals of this size were seen on the bar and perhaps 

 half were attached. They wei-e not observed in the act of crawling but they are ea])able 

 of swimming actively as later described. On account of the size of the specimens it 

 was easy to observe the process of formation of the byssal fixation. In observing it, 

 the scallops should be confined in a dish as every mov^ement is then clearl)^ perceptible. 

 Lying on the I'ight valve, the foot is extended on the surface of the dish, the flattened 

 distal portiou taking a firm hold as if about to crawl. This position is maintained for a 

 moment or two and then the foot is withdrawn within the body, I)y the motion of retrac- 



