1888.] 533 [Jackson. 



groups, and it is believed that the later development of the oyster 

 throws light on the problem. 



DEVELOPMENT OF THE SOFT PARTS. 



The early embryological development of the oyster may be fol- 

 lowed in the writings of Dr. R. Horst (6, 7), on the European oys- 

 ter, 0. edulis L. and Professor W. K. Brooks (4) on the development 

 of our oyster, 0. virginiana Lister. The later stages of develop- 

 ment have been studied by Professor Huxley (8), and Dr. R. 

 Horst (7), in the European oyster, and Professor J. A. Ryder, 

 in our species. 



The lamellibranchiate identity of the embryo is first emphatically 

 marked when two valves are seen to have resulted from developing 

 growth of the preconchylian gland (Horst's (7) fig. 14). The mouth 

 and anus, when developed, arise in close proximity on the ventral 

 aspect of the embryo (Brooks' (4) fig. 38), a generalized molluscan 

 embryo characteristic. The velum which existed before this stage 

 is clearly marked off, lying directly above the mouth. 



A little later (Horst's (7) fig. 15), the development of the first 

 adductor muscle begins in the middle anterior part of the body, 

 and close to the dorsal margin of the velum. The retractor mus- 

 cles of the velum and liver develop. The anus revolves in the 

 plane of the valves through a considerable angle toward the dorsal 

 aspect of body (see Brooks' (4), figs. 38-44). The intestine by in- 

 terstitial growth makes a single loop-like curve on the left side ; 

 this with some other slight changes brings us to the stage figured 

 by Professor Huxley (8) (see this paper fig. 1, pi. iv). Ryder (23), 

 in remarking on this stage, notes that the intestine is already flexed 

 on itself, in much the same manner as in the adult, though it does 

 not extend as far anteriorly, and its position is modified later by 

 flexions of the stomach. 



Professor Huxley (8), in speaking of the single adductor, says: 

 " It is a very curious circumstance that this adductor muscle is 

 not the same as that which exists in the adult. It lies in fact in 

 the fore part of the body, and on the dorsal side of the alimentary 

 canal. The great muscle of the adult, on the other hand, lies on 

 the ventral side of the alimentary canal and in the hinder part of 

 the body, and as the muscles, respectively, lie on opposite sides of 

 the alimentary canal, that of the adult cannot be that of the larva 

 which has merely shifted its position ; for in order to get from one 



