802 ROBERT TRACY JACKSON ON THE 



anterior adcluetor l)C"ing developed fii'st. These are MN'tiliis and Unio. As the nearly re- 

 lated Modiola and Anodun lollow tliii rulethesu two genera may he eonsideix-d as excep- 

 tions, or ])erhaps illustrate eri'ors of ohservation us discussed in my preliminary paper, 

 (35), p. 535. 



In Ostrea the mouth and anns develop ventrally, the anal extremit}' of the intestine re- 

 volves dorsally and the anterioi- adductoi- is developed. Later, the posterioi- 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 jNIytilus and 

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

 of |)lan. Tlie 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 iu Pelecypods as the typical mode, and the development of 

 the ])Ostei'ior adductor later, after the intestine has revolved into place, from its early ven- 

 tral position. This theoiy easily explains the conskint I'clative ])ositions of the mouth 

 and anus to the two adductors in the adults of dimyarians, or to the single adductoi-, 

 Avhere only one exists, as in monomyarians. 



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

 bryonic oyster. First, a single-muscled stage, the early monnmyarian, fig. 25, p. 300, 

 pointing towards a ])roblemutical ancestor, which iu the adult condition had only one ad- 

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

 two-muscled stage, dimyarian, PI. xxiv, figs. 1-2, pointing to an ancestral 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 dimyarian, 

 adult ancestor ])ossessed a velum. It exists here ])resumably because of the lapping 

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

 bryonic develoi^ment of the oyster. The shell which covers these stages is continuous 

 in outline. 



As stated, a velum still existed at this dinwarian stage, PI. xxrv, 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 beyond the margins of the shell. Attempts have 

 been made by Horst and Ryder (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 effected 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. 24, p. 299, and I 

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

 veloped in the ventral poi-tion 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 tliat tlic first formed muscle does not - The fry described is comparalile in asje to my PI. xxiv, 



point to an ancestral single-muscled Telecypod, in which (i^s. 13-lG, and is not to be confounded in its attacliment 



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



to some muscle iu the primitive ancestral mollusc from prodissoconch stage, PI. xxiv, fig. 17. 

 which the Pclccypcda were derived, and which muscle be- 

 came the anterior adductor of the Pelecypoda. 



