1888.] 537 [Jackson, 



one adductor muscle, and that in the anterior portion of the body. 

 The figure of this stage shows the early formed velum to be still 

 existent, but it is not likely that the early antemonomyarian adult 

 ancestor possessed a velum. It exists here presumably because 

 of the lapping over of stages, due to concentration of develop- 

 ment, which was noted as being most marked in early embryonic 

 development. The second stage is the two-muscled stage, dimya- 

 rian, fig. 2, pi. iv, pointing to an ancestral adult form which had 

 two muscles like the typical dinryarian. The shell which covers 

 these stages is continuous in outline, and I have not made out any 

 indication of change of form or structure to coincide with the 

 change in the soft parts. 



The next stage of a developing oyster is shown in fig. 3, pi. iv. 

 This is a younger spat stage than has been previously figured, but 

 shows great changes from the last, fig. 2, pi. iv. No trace of an 

 anterior adductor was made out, but the posterior adductor can 

 be seen just on the limits of the prodissoconch. Palps and gills 

 were both seen and they have revolved dorsally from their previous 

 ventral position. The gills are filamentous in structure, similar 

 to those described in young Mytilus by Lacaze-Duthiers (15). In 

 the specimen from which my figure was drawn, the outer limits of 

 the filaments appeared to be free ; but this may not have been the 

 case, as it was difficult to see through the somewhat opaque shell. 

 In fig. 4, pi. iv, drawn from a living oyster of a little older growth, 

 the tips of the filaments are joined b}' a continuous, connecting 

 membrane, similar to that figured in older stages of Mytilus by 

 Lacaze-Duthiers (15). 



The intestine wraps around the adductor muscle, as in fig. 2, 

 pi. iv, and in older stages, fig. 18, pi. vn. The mouth parts have 

 revolved dorsally, yet not through so great an angle from the posi- 

 tion they formerly occupied in fig. 2, pi. iv, as seen in the adult, 

 fig. 18, pi. vn. Professor Ryder (23) remarks, in speaking of a 

 similar figure of a spat, which was taken from a later stage of 

 growth, that the mouth opens downwards and not so directly for- 

 wards as in the adult ; evidently it had not yet completed its up- 

 ward revolution. 



I succeeded in growing spat on glass slides, and excellent oppor- 

 tunities for studying them alive, under the microscope, were con- 

 sequently at my command. When undisturbed and quietly feeding, 

 the mantle stretches out to the margin of the shell on all sides, and 



