304 ROBERT TRACY JACKSON ON THE 



beaker of sea water for four days. When twenty-four hours older than the figures cited, 

 a good deal of s])at growth had taken place; the velum had disappeared and the anterior 

 adductor was difficult to discern, but I think still existed in a position much nearer the 

 liiTige margin than in figs. 1-2; the muscle had thei"efoi-e revolved dorsally. The pos- 

 terior adductor had not changed much in its position; but the palps had i-evolved dorsally. 

 The animal was j^ellowish-green as on the previous day, and the mantle border was 

 wide and actively extended and retracted. The action of the parts was more clearly seen 

 when viewed from the left side through the glass to which it Avas attached than from the 

 right iipper side. From this view many observations were made. The gill filaments at 

 the end of twent3'-four hours were not yet joined to one another on their ventral mar- 

 gins as in later stages. 



When the same specimen was forty-eight hoirs oldei* than PI. xxiv, figs. 1-2, no an- 

 terior adductor muscle was seen. The postei-ior adductor from its eai'lier position had 

 moved ventrally so that it lay near the border of the prodissoconch valves. The palps 

 had revolved dorsally so that they were situated near the hinge line, but Avere directed 

 ventrally; a fact to be discussed later. I did not see the radial muscles of the mantle 

 visible in later stages, but the}' donl)tless existed; no marginal tentacles were yet devel- 

 oped. The rhythmical pulsating body, noticed on the first day in the umbo of the left 

 valve, had disappeared or become hidden in the increasing liver follicles. No heart 

 was discovered on the dorsal border of the adductor, where it is later found. The liver 

 was greenish brown, not light green, as at first. 



PI. XXIV, fig. 3, is from the same specimen three days older than figs. 1-2. The spat 

 shell has grown considerably, the posterior adductor has moved to a more ventral posi- 

 tion than it occupied forty-eight hours after attachment, and the palps have moTed far- 

 thei" dorsally. The gill filaments are now connected on their ventral margins by cross 

 connecting bars, and this is the beginning of the intricate s_ystem of cross connecting 

 network, characteristic of the adult ostrean gill. The filaments in young and adult 

 Pecten are joined by intei-locking but separable cilia, similarly to those of Mytilus as 

 described by Peck, but the connection in Ostrea is a complete oi'ganic union brought 

 about by the concrescence of fleshy processes from the margins of adjoining filaments. 

 The cross bars begin to be developed between the filaments in the anterior portion of the 

 gill and from that I'egion are formed posteriorly in a serial manner between successive 

 filaments. In oysters np to 2.2 mm. in height only a single series of cross bars exist 

 and those are on the ventral margin as in PI. xxiv, fig. 3. The ends of the gill fila- 

 ments in the spat, fig. 3, are recurved and joined by concrescence at their tips with the 

 recurved filament tips of the opposed gill-lamella. This is understood by comparing the 

 diagi-ams of the gills of developing Ostrea of this stage, PI. xxiv, fig. 7, with those of an 

 adult Pecten, PI. xxiv, fig. 12. The contiguous surfaces of the direct and reflected 

 portion of the filaments ai-e not organically connected at this early period, so that each 

 gill-lamella represents a hollow cavity bordered b}' its filamentous walls and a condition 

 similar to this has been observed in adult Pei-na. The two gills are of about equal size 

 at this period. Ilorst states that in young Ostrea edulis the gills ai'c filamentous, joined 

 only at the bases and tips; and he compares this condition with Lacaze-Dnthiers' (41) 

 well-known obsei'vations on Mytilus. 



