300 



ROBERT TRACY JACKSON ON THE 



adductor must be developed and this early anterior adductor lost in later development. 

 This was all that was known concerning the development of the adductor muscles of 

 the oyster when I published my preliminary paper 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 neai-ly disc-shaped, but flattened 

 at the hinge area. A ciliated velum existed, and as Ryder noted, 

 " the embryos appeared to be attached to the glass by a marginal 

 extension of the mantle over the edge of the lower valve, forming 

 a sort of lip," but they were not firmly fixed as ''the embryos 

 swung to and fro." He did not succeed with later attempts in 

 getting similar attachments, and from my own observations I 

 have every reason to believe that tlie fixation was of a transitory 

 nature, not to be confounded with the fixation which young oys- 

 ters later acquire when they become fixed for life. 



I have thus f (17' considered the earhj develo'pment of the oyster, 

 using facts observed by other investigators. This is done in order 

 to mal:e clear the later portion of the embryological development 

 and also to bring out some facts elucidating the development of 

 the group of Pelecypoda. All later observations are my oivn, ex- 

 cepting where specially credited. 



Between the stage fig. 25, and our next stage, PI. xxiv, figs. 

 1-2, there is a blank in the knowledge of the development of the 

 oyster. It has not been described in the European species, and 

 all attempts to obtain it in onr species have failed. In artificial 

 confinement the oyster dies at this stage. 



The next step in the anatomy which I have to consider is that 

 of the oyster when it has completed the prodissoconch growth 

 and just preceding the beginning of the dissoconch, or "spat 

 o-rowth." This period is the last of the embryonic stages, using as bases of consideration 

 the structure of the soft parts and the hai'd as well as a comparison with a similar period 

 in other developing Pelecypoda. In my preliminary paper (36) I showed the important 

 fact that the oyster at this stage has two adductor muscles, situated relatively to the ali- 

 mentary canal as they are in adult dimyarian Pelecypods. The specimen then figured 

 was from alcoholic material collected at Buzzards Bay, and the parts had contracted so 

 that some features were not successfully made out; the muscles however were i-endered 

 clear by treating the specimen with dilute acetic acid, and thus dissolving the calcareous 

 shell. Living material of this age was obtained later growing on glass slides, so that it 

 could be studied more satisfiictorily. 



On the third of August, 1888, clean glass was put into a drain-pipe trap on the sand- 

 bar. The following day it was examined and a young oyster, PI. xxiv, figs. 1-2, was 

 found attached. No spat growth had taken place, but the embryo was firmly attached 

 by the ventral margin of the left valve. On driving a curi-ent of water foix-ibly over 

 the glass from a pipette it did not rock or move from its position, thus demonstrating 



Fig. 25. 



EMBKYO OF OSTREA EDULIS 



L. (AFTER UUXLEY). 



The phylembryo stage, 

 viewed from the left side, a, 

 ad, anterior adductor muscle, 

 which alone exists at this 

 stage ; m, mouth ; <e, oesopha- 

 gus ; a, anus ; i, intestine ; st, 

 stomach ; 11, left lobe of the 

 liver; v. velum; rs, ri, supe- 

 rior and inferior muscles, 

 ■n-hicli retract the velum into 

 the shell, sh ; the hinge, h, is 

 straight. Compare with PI. 

 xxui, figs. 7-11 of phylem- 

 bryonic stages of developing 

 Pelecypods, where as here the 

 hinge line is straight and the 

 anterior adductor aloue ex- 

 ists. 



