180 Capt. DuCane on the Metamorphods of Crustacea. 



itself of this last envelope and got rid of its natatory legs, 

 than the subabdominal fins, which have hitherto been un- 

 formed and useless, come out ornamented with a delicate hair- 

 like fringe, and become the organs by which the prawn ad- 

 vances in the water, and which are kept constantly in the same 

 rapid motion that the natatory legs were kept in whilst the ani- 

 mal was in its larva state. The animal henceforth also ceases to 

 move backw^ards, excepting for the pui-pose of avoiding danger. 



Nos. 7. and 8. are drawings of the larva of the common 

 shrimp {Crangon vulgaris) \ the larvae were kept seven days 

 from the time of their excliision from the egg, and were then 

 destroyed in consequence of my leaving home; they had at that 

 period undergone no change. The general character is the 

 same as the larva of the prawn, but they have in this stage 

 only three pairs of natatory legs; and it is remarkable, that their 

 movements, instead of being retrograde like the larvae of the 

 prawn, are constantly rotatory, excepting when they come in 

 contact w ith each other : they then dart suddenly off in a la- 

 teral direction ; the rudiments of the true legs were visible, 

 but too minute to be enumerated. 



The above particulars, following up as they do the pro- 

 gressive changes in the prawn, and confirming the valuable 

 observations of Mr. Thompson as to the fact of the macrourous 

 decapods being subject to metamorphosis, will I trust be ac- 

 ceptable to the Association, and excuse my troubling you in 

 such detail. 



I have the honour to be. Sir, 



Your veiy obedient Servant, 



Rev. Leonard Jenyns, C. DuCane. 



^c. ^c. ^c. 



Extract from a Letter on the same subject from Capt. 

 DuCane, R.N., to W. S. MacLeay, Esq. 

 The larva: mentioned in the paper, unfortimately sent too late 

 to the British Association, were taken from a salt-w-ater ditch 

 in this neighbourhood. I have since hatched the ova of some 

 ditch prawns in pure fresh water, although I had previously 

 kept them upwards of a month. The larva on quitting the 

 egg corresponded precisely with my draAvings Nos, 1, and 2. 



