[9] DEVELOPMENT OF THE OYSTER. 787 



as spat, may be observed upon comparing together Figs. 1 and 2; Fig. 1 

 shows a larva viewed from the left side ; Fig. 2 represents the spat as 

 seen from the right side, but in both the course of the intestine is 

 displayed. In Fig. 1 the single loop of the intestine * does not extend 

 nearly as far forward anteriorly as in Fig. 2, it is therefore evident that 

 during the metamorphosis this loop is prolonged so that in the adult it 

 actually crosses the gullet, but the intestinal canal as a whole remains 

 flexed upon itself in much the same manner from the later larval stage 

 onwards with its anterior flexure thrown forward over the left side of 

 the stomach. The posterior end of the stomach, together with the first 

 flexure of the intestine is afterwards considerably depressed, while the 

 oesophagus is thrown upwards and between the first flexure of the in- 

 testine and the rectum, the permanent posterior adductor muscle a 

 (Fig. 2) is developed, A^ery probably from wandering cells which have 

 dehisced from the visceral cavity or blastocoel of the embryo. 



Upon comparing the two figures it would appear as if the mouth o. 

 Fig. 1, together with the oesophagus and forepart of the stomach, woukl 

 have to be rotated through an angle of nearly ninety degrees in order 

 to bring it into the relation with the hinge /«, as shown at o, in Fig. 2. 

 This alteration in the relative positions of the viscera during the pas- 

 sage of the larva into the adult condition is one of the most striking 

 changes which occur during the metamorphosis. 



One of the most conspicuous differences between the symmetrical 

 larva and the young spat is the absence of gills in the former and 

 their presence in the latter. These grow out as blunt fleshy processes, 

 behind the mouth o, and in front of the anus a (Fig. 1), after a pallial 

 sinus has been formed in that position. Sections which I have prepared 

 of very young spat seem to show that the development of the branchise 

 is not completed until some time after the fixed and spat stage has been 

 assumed. Cross-sections of very young spat one-eighth of an inch in 

 diameter show only two gill pouches developed posteriorly, instead of 

 four, as in the adult ; this would indicate that the outer gill pouches are 

 formed during the young condition of the spat, and some time after the 

 symmetrical larval condition has been passed. As far as the branchial 

 system is concerned it therefore appears evident that it is completely 

 developed after the true larval condition is over, and the metamorphosis 

 is otherwise comj)lete. 



The liver, according to the testimony of a number of investigators, 

 arises as a pair of hollow outgrowths on either side of the stomach. 

 It seems therefore to develoi> from bilaterally symmetrical rudiments 

 like the shell, and that its subsequently more complex structure is a 

 result of secondary or later processes of growth, affecting mainly the 

 walls of the original right and left hepatic lobes. These hollow lobes 

 seem to arise rather from the lower lateral portions of the gastric dilata- 

 tion of the alimentary canal of the embrj^o, and traces of this original 

 symmetry }|r§ not wawtiug when we come to observe the relations of the 



