Development of Estheria Packardi. 21 



increased considerably in size, measuring now 0,86 mm. in 

 length. The enclosed animal does not much differ from that 

 in the preceding stage, and is still highly pellucid, with only 

 a very faint yellowish tinge, the shell being, as in that 

 stage, almost colourless. 



Fourth postlarval stage. 



(PL III, fig. 3). 



The shell in this stage masures 0,93 mm., and exhibits 

 3 lines of growth, the outer 2 being, like the free edges ot 

 the valves, fringed with delicate bristles. Its form has be- 

 come somewhat more elongated in proportion to its height, 

 but is still rather unlike that of the adult animal. 



Of the several appendages, the antennulæ have some- 

 what increased in length, but are still quite simple, without 

 any distinct segmentation or lateral lobules. The rami of 

 the antennæ have likewise become more elongated, and are 

 each divided into 7 well-defined joints. Of branchial legs, 

 10 pairs are now distinctly developed, and on the caudal 

 plates 3 denticles have appeared above the spiniform lower 

 corners. 



As to the inner organs, it is only to be noted, that the 

 cæcal appendages of the intestine have commenced to divide 

 in a dichotomic manner, to form the complicated liver-masses 

 filling up, in the adult animal, a great part of the inner 

 cavity of the head. 



Seventh postlarval stage. 



(PL IV, fig. 1). 



In this stage the shell exhibits 5 well defined lines ot 

 growth, and measures in length 1,70 mm. The primitive 



