Phylogeny of the Pelecypoda : the Aviculidae and their allies . side; this with some other slight changesbrings us to the stage, fig. 25, described by Huxley (2(5). Ryder (63), remarking onthis stage, notes that the intestine is already flexed on itself in much the same manneras in the adult, though it does not extend as fiir anteriorly, and its position is modifiedlater by flexions of the stomach and oesophagus. The shell of the stage, fig. 25, is symmetrical, equivalvular, and of about the period ofdevelopment of PI. xxiv, figs. 13-16, of the develoi^ing shell of this same species. Themouth
Phylogeny of the Pelecypoda : the Aviculidae and their allies . side; this with some other slight changesbrings us to the stage, fig. 25, described by Huxley (2(5). Ryder (63), remarking onthis stage, notes that the intestine is already flexed on itself in much the same manneras in the adult, though it does not extend as fiir anteriorly, and its position is modifiedlater by flexions of the stomach and oesophagus. The shell of the stage, fig. 25, is symmetrical, equivalvular, and of about the period ofdevelopment of PI. xxiv, figs. 13-16, of the develoi^ing shell of this same species. Themouth has yet no palps, a sac-like process of the stomach on either side representsthe liver, two pairs of muscles serve to retract the velum, and the single adductor musclecloses the valves. Huxley observes that this early anterior adductor cannot be theeqiiivalent of the single adductor of the adult as it is on the ventral side of the alimen-tary canal whereas that of the adult is on the dorsal side. He says that a second posterior 300 ROBERT TRACY JACKSON ON THE. adductor must be developed and this early anterior adductor lost in later was all that was known concerning the development of the addnctoi- muscles ofthe oyster when I published my preliminary papei- in which it was shown that two adduc-tors exist at the completed prodissoconch stage. A stage of 0. virginiana of similar age to our fig. 25 of O. edulis has been figured by Ryder (62). The valves were nearly disc-shaped, but flattenedat the hinge area. A ciliated velum existed, and as Ryder noted, the embryos appeared to be attached to the glass by a marginalextension of the mantle over the edge of the lower valve, forminga sort of lip, but they were not firmly fixed as the embryosswung to and fro. He did not succeed with later attempts ingetting similar attachments, and from my own obseiwations Ihave every reason to believe that the fixation was of a transitorynature, not to be confounded with the fixation
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