Development of Estheria Packardi. 19 



less. Its length, measured from the tip of the rostrum to 

 the end of the caudal claws, is 0,65 mm , or about the same 

 as in the last larval stage. The length of the shell scarcely 

 exceeds 0,40 mm. 



As to the mode in which the animal moves through the 

 water, it looks very diflerent from that observed in the 

 larva. Whereas in the latter, by rhythmical and not very 

 rapid strokes of the antennæ, and partly also of the mandi- 

 bular feet, a rather slow and somewhat jumping motion is 

 effected, it has now more the character of an even run, the 

 animal being propelled by very rapid strokes of the outer 

 part of the antennæ alone. At times the animal is also seen 

 affixing itself with its back to the walls of the aquarium, 

 but both in this case and during swimming, the branchial 

 legs are found to be in an uninterrupted swinging motion. 



Second posllarval stage. 



(PL Ill, fig. 2). 



This stage is chiefly characterized by the appearence on 

 the shell of the 1st line of growth. This line is nothing 

 less than the edges of the primitive valves, which, as it 

 were, are superposed on the newly formed ones, so as to 

 leave only the peripheric part of the latter exposed. This 

 part is rather narrow, and has the edges throughout fringed 

 with delicate bristles, not found on the edges of the primitive 

 valves. By the said additional peripheric area, the shell has 

 increased considerably in size, so as now to allow the animal, 

 by a strong flexion of its anterior and posterior parts, to 

 be completely withdrawn into the shell. Its form is, how- 

 ever, nearly unaltered. 



