Development of Estheria Packardi. 23 



very small part of the shell, and it is easily seen that the 

 primitive valves become at last restricted to the tip of the 

 umbones, where, on a closer examination, they can be dis- 

 cerned even in full-grown specimens. 



The enclosed animai (not dravn in the figure) has now 

 assumed a light brownish hue, but is still rather transparent, 

 so as to admit of the alimentary tube being very distinctly 

 traced in its whole length. Also the shell-gland, which in 

 the adult animal is very difficult to observe, appears rather 

 distinctly inside the umbonal part of the valves. Of branchial 

 legs may now easily be counted 16 pairs, and behind them 

 some other pairs are apparently in process of formation. 



Subsequent postlarval stages. 



The succeeding stages, which are rather numerous, I 

 do not find it neccessary to describe in detail. They are 

 characterized by a very gradual increase both of the shell 

 and animal, the lines of growth at the same time augmenting 

 in number, and the several appendages assuming gradually 

 the appearance characteristic of the adult animal. In most 

 of the stages the setous armature of the lines of growth and 

 of the free edges of the valves could be discerned; but the 

 bristles gradually become shorter and partly broken off, 

 so as atlast, for the most part, only to leave their insertions. 

 The peculiar radiating striation of the shell described in my 

 previous paper, is undoubtedly due to these bristles, or more 

 correctly to their former insertions on the edges of the several 

 superposed valves. 



