324 BULLETIN OF THE 



opment of Astacus, Rathke came to the conclusion that the prawn too 

 acquires, while still within the egg, all the external parts (excepting the 

 sexual organs) which are found in the adult, and that it is subject to no 

 greater structural changes after leaving the egg than a bird is ! After 

 further researches upon the development of Decapods, Rathke acknowl- 

 edged his error, and his handsome tribute to J. V. Thompson, in this 

 connection, is a notable example of candor.* 



The changes which the young suffers after leaving the egg were first 

 observed by J. V. Thompson. In the summer of 1828 he obtained the 

 first stage of the larva by hatching the eggs of Palcernon serrahis, and in 

 a paper published in 1836 t gave a rough figure of this, together with 

 two older larval prawns which he supposes to belong to the same species, 

 although they were captured free-swimming. 



How much allowance must be made for inaccuracy in Thompson's 

 figures it is hard to say. The first stage is represented with but two 

 pairs of cleft members ; in the second stage the larva has acquired an 

 additional pair of swimming-feet, the sixth pair of abdominal appendages 

 are present, and form with the telson a terminal fin about like that in 

 our third larval stage. There is yet no dorsal spine on the carapace, 

 and the first pair of antennae are still simple. 



The third stage apparently represents a phase somewhat more ad- 

 vanced than our sixth stage. The carapace is now armed with three 

 spines on the median dorsal line, and a supra-orbital spine on each side. 

 All of the appendages are now present, the chelae are well formed, and 

 all of the thoracic members, not excepting the posterior pair, are fur- 

 nished with a natatory branch. The flagellum of the second pair of 

 antennae is in the condition of the adult, being divided into an indefinite 

 number of annuli. In other respects the young prawn agrees pretty well 

 with .the sixth stage of Palcemonetes. 



The most complete account of the larval stages of Palcernon up to the 

 present time is that of Captain DuCane, published in 1839, in the second 



* Zur Entwickelungsgeschichte der Dekapoden. Arch. f. Naturgesch., 1840, I. 

 p. 248. 



Beitrage zur vergleichenden Anatomie und Physiologie, Reisebemerkungen aus 

 Skandinavien, p. 46. Neueste Schriften der Naturforsch. Gesell. in Danzig, Vol. III. 

 1842. 



t Memoir on the Metamorphosis in the Macrourse or Long-tailed Crustacea, exem- 

 plified in the Prawn (Palsemon serratus). Edinburgh New Philosoph. Jour., Vol. 

 XXI. pp. 221-223, PI. I. 1836. A short notice of this paper appears in the "Ab- 

 stracts of the Papers printed in the Philosophical Trans, of the Royal Soc," Vol. 

 III. p. 371. 1836. 



