302 ROBERT TRACY JACKSON ON THE 



anterior adductor being developed first. These are IMytihis and Unio. As the nearly re- 

 lated Modiola and Anodon follow the rule these two genera may be considered as excep- 

 tions, oi- perhaps illusti-ate errors of observation as discussed in my preliminary paper, 

 (35), p. 535. 



In Ostrea the month and amis develop ventrally, the anal extremity of the intestine re- 

 volves dorsally and the antei'ior adductor is developed. Latei', the posterior adductor de- 

 velops on the ventral side of the intestine. "When one compares this with the figures and 

 descriptions of the development of the above-mentioned genera (excepting Mytilus and 

 Unio), together with the anatomy of the adults, it seems that there is a close uniformity 

 of plan. The genera mentioned are not numerous, but they are taken from widely sep- 

 arate groups of Pelecypoda. There is then strong evidence in favor of the development 

 of the anterior adductor first in Pelecypods as the typical mode, and the development of 

 the posterior adductor later, after the intestine has revolved into place, from its early ven- 

 tral position. This theory easily explains tlie constant relative ])ositions of the mouth 

 and anus to the two adductors in the adults of diniyarians, or to the single adductor, 

 where only one exists, as in monomyai ians. 



It is seen that there are two distinct stages of development of the muscles of the em- 

 bryonic 03'ster. First, a single-mnscled stage, the early monnmyarian, fig. 25, p. 300, 

 pointing towards a problematical ancestor, which in the adult condition had only one ad- 

 ductor muscle, and that one in the anterior portion of the body.^ The second stage is the 

 two-muscled stage, dimyarian, PI. xxrv, figs. 1-2, pointing to an ancesti'al adult form 

 which had two muscles like the typical dimyarians. The figure of this stage shows the 

 early formed velum to be still existent, but it is not supposed that the early dimyai'ian, 

 adult ancestor possessed a velum. It exists here presumably because of the lapping 

 over of stages, due to concentrated development, which is most marked in the early em- 

 br3'onie development of the 03'ster. The shell which covers these stages is continuous 

 in outline. 



As stated, a velum still existed at this dimyarian stage, PI. xxiv, figs. 1-2. The long 

 vibi-ating cilia were in active motion and some motion of the velum as a whole was 

 noted; but it was not seen to extend be3'ond the margins of the shell. Attempts have 

 been made by Ilorst and Rydei' (62) to discover the method by which the young oyster 

 attached itself to the object of permanent fixation and both investigators surmised that 

 it might have a brief byssal attachment. Ryder, in considering the fixation of the em- 

 bryos described by him," thought that it was etfected by means of the border of the man- 

 tle reflected over the edge of the undermost valve as shown in his figure. The nearest 

 approach to a foot known in the developing oyster is that shown in fig. 21, p. 299, and I 

 discovered no traces of a foot in my yoimgest specimens. As the byssws is an organ de- 

 veloped in the ventral portion of the foot, the high reduction or almost complete absence 

 of that organ is in itself strongest evidence against the suggestion that the attachment 



' It is possible that the first formed muscle does not ° The fry described is comparable in age to mj- PI. xxiv, 



point to an ancestral single-muscled Pelecypod, in which flgs. 13-16, and is not to be confounded in its attachment 



only the anterior adductor was developed. It may point with the permanent attachment made at the close of the 



to some muscle in the primitive ancestral mollusc from prodissoconch stage, I'l. xxiv, fig. 17. 

 which the Pelecypoda were derived, and which muscle be- 

 came the anterior adductor of the I'elecypoda. 



