Jackson.] 534 [April 4, 



side of the alimentary canal to the other, it must needs cut through 

 that organ. But, as in the adult, no adductor muscle is discover- 

 able in the position occupied by that of the larva, or anywhere on 

 the dorsal side of the alimentary canal, while on the other hand 

 there is no trace of any adductor on the ventral side, in the larva, 

 — it follows that the dorsal or anterior adductor of the larva must 

 vanish in the course of development, and that a new ventral or 

 posterior adductor must be developed to play the same part and 

 replace the original muscle functionally, though not morphologi- 

 cally." He also notes that the cockle has at first, like the young 

 oyster, only one adductor, which answers to the anterior of the two 

 adductors which the cockle possesses in the adult. This is up to 

 the present time all that has been known concerning the adductor 

 muscles of the young oyster. 



The next stage, fig. 2, pi. iv, in the development which I have to 

 consider, is that of an oyster which had completed its prodissoconch 

 and a very slight spat growth had taken place on the ventral mar- 

 gin. The specimen, preserved in weak alcohol, was removed lately 

 from cultch on which it had grown in Buzzards Bay during last 

 summer. The calcareous portion of the shell was dissolved with 

 dilute acetic acid, and the main features of the anatomy could then 

 be seen. The velum had disappeared. The mouth from its pre- 

 vious ventral position (shown in fig. 1, pi. iv), had revolved dor- 

 sally, in the plane of the edges of the valves, similarly to theanus, 

 but in the opposite direction, and had stopped beneath the still ex- 

 istent first-formed adductor muscle, fig. 2, pi. iv. This muscle ap- 

 parently checked its farther revolution dorsally. Continuous with 

 the mouth is the oesophagus which is directed dorsally. Imme- 

 diately below the mouth is a large organ, which is probably the 

 developing palps. The gills are developed considerably on the 

 ventral aspect. The chief and most interesting feature of this 

 stage, fig. 2, pi. iv, is the existence of a second adductor muscle, 

 in the posterior portion of the body, of about the same size as the 

 anterior adductor. This muscle occupies a position below the 

 terminus of the intestine, which organ wraps around the posterior 

 aspect of the muscle, as it does in later stages, and in all adult 

 Lamellibranchs. 



We have then the interesting feature of a monomyarian Lamel- 

 libranch, in its young stages possessing two muscles, situated 

 relatively to the alimentary canal as they are in typical adult di- 



