Jackson.] 538 [April 1, 



even protrudes considerably beyond it. It moves actively, being 

 constantly retracted and extended by the radial muscles 1 described 

 by Professor Ryder (23). The marginal tentacles are also con- 

 stantly in motion, being continually elongated and retracted. 

 When elongated they are used with a tentative, feeler-like motion, 

 and if irritated are capable of extreme prolongation and then ex- 

 hibit great activity. The gills and palps move frequently and sud- 

 denly. 



My studies corroborate Professor Ryder's figure of a specimen 

 of a spat stage in all general points, except the convolutions of the 

 intestine, which I did not succeed in making out. 



During later stages of growth, the soft parts rapidly assume the 

 positions which they occupy in the adult, fig. 18, pi. vn. The mouth 

 parts are revolved towards the umbos, and the anus and adductor 

 are revolved in the opposite direction, towards the free ends of the 

 valves, until they attain a position at an angle of about forty-five 

 degrees from the position which they had in the two-muscled stage, 

 fig. 2, pi. iv. 



In the dimyarian stage of the young oyster, fig. 2, pi. iv, the rel- 

 ative position of the axes of the body to the hinge axis of the 

 shell 2 resembles an adult dimyarian Lamellibranch, fig. 15, pi. vn. 

 The hinge is dorsal, free edges of the valves ventral, the mouth and 

 anus lie at either end of the equal valves, so that the anteroposte- 

 rior axis 3 is nearly parallel to the hinge axis. In the adult oyster, fig. 

 18, pi. vn, this relation is different. The mouth lies close up under 

 the hinge line, marking this as the anterior end of the body. The 

 free ends of the valves are at the posterior extremity, the gills lie 

 on the ventral side, and the intestine on the dorsal. 4 The antero- 

 posterior axis passes nearly through the umbos and centre of the 



1 In my fig. 3, pi. iv, the radial muscles were not seen ; but they are readily seen in 

 living specimens from which the shell has been partially broken away. 



2 The hinge axis refers to an ideal line drawn through the hinge area, and coinciding 

 witli the axis of motion of the valves, as shown in figs. 15-18 inclusive, pi. VII. My at- 

 tention was called to the value of this line in considering the theory of the revolution 

 of the axes, by Mr. B. H. Van Vleck of the Boston Society of Natural History. 



3 The anteroposterior axis, as shown in figs. 15-18, pi. VII, is considered as passing 

 through the mouth and middle of the posterior adductor muscle and nearly or quite 

 coinciding with the termination of the intestine. 



4 The fact that the umbos in the oyster are at the anterior end of the body has al- 

 ready been pointed out by several authors, and Hyatt (10) figures an oyster drawn over 

 a clam to show their relations. Ryder (23), page 787, notes the rotation of nearly ninety 

 degrees that must take place from my fig. 1, pi. IV, after Huxley, to bring about the 

 adult relation of parts. 



