MUSEUM OF COMPAKATIVE ZOOLOGY. 310 



sented by Fig. 15 of Plate IV. Tliis was frequently taken in the hand- 

 net free-swimmiug at the surface of the sea. Although but eight milli- 

 metres long, the larva has acquired the form and almost every charac- 

 ter of the adult prawn. 



Among the drawings of Crustacean larvoe made by Mr. Agassiz at 

 Newport, which he has generously placed at my disposal, are some 

 details of the structure of a larval stage of Palcemonetes vulgaris which 

 happily fills in some measure the gap between the si.\th larval stage and 

 the adult form. This phase in the development I will designate as the 



Seventh Larval Stage (PI. IV. Figs. 9- 14). — There is no sketch of 

 the general form of this stage among Mr. Agassiz's drawings. The 

 rostrum (PI. IV. Fig. 9) is long and scymitar-shaped, with six teeth 

 above and two below. Between the teeth of the dorsal side are hairs, 

 as in the adult. The anterior third of the rostrum is destitute of teeth. 

 The carapace is armed, moreover, with an antcnnal and a minute bran- 

 chiostegal spine on each side. The supra-orbital spine is reduced to a 

 mere rudiment. 



The natatory branch of the maxillipeds and legs (PI. IV. Figs. 10, 11, 

 13, r e) is reduced to a short style composed of one segment. In this 

 we see an interesting transition state between the Schizopod and the 

 Decapod leg. The inner branches of the first and second pair of max- 

 illipeds have also diminished, and now consist of one and two segments 

 respectively (PI. IV. Figs. 10, 11). The segments of the internal 

 branches of the chelipeds and ambulatory appendages are clearly seven 

 in number. The last pair of thoracic legs (PI. IV. Fig. 14) is of course 

 without the external styliform appendage. 



The antennae, swimmerets, and telson are not represented in Mr. 

 Agassiz's figures, nor is the natural size indicated. 



Specimens in the sixth larval stage may measure eight millimetres 

 from the tip of the rostrum to the tip of the telson. This is as long as 

 specimens which have attained the adult form, such as is represented by 

 Figure 15 of Plate IV. Hence I infer that but few stages intervene. 

 It is highly probable, however, that the larva suffers one or two moults 

 between the seventh stage and the attainment of the shape of the adult 

 prawn. 



Early Stage of the Adult Form (PI. IV. Fig. 1 5). — Although measuring 

 only eight millimetres from the tip of the rostrum to the end of the 

 telson, the prawn has now the true Decapod structure ; although still 

 a free-swimming surface-dweller. Every trace of the external swimming- 

 branches of the five pairs of legs has vanished, and the structure is in 



