MUSEUM OF COMPARATIVE ZOOLOGY. 323 



penultimate. All the thoracic appendages, excepting the last pair, are 

 furnished with external natatory branches. By subsequent moults the 

 natatory branches are reduced in size, and finally disappear altogether. 

 During the evolution of the thoracic Umbs, the abdominal appendages 

 make their appearance, first the posterior pair (which develop within 

 the caudal plate or tclson), then the anterior pairs. 



The different forms which the individual assumes during its develop- 

 ment are in this case the result of gradual growth, each successive 

 moult developing a form which shows but a slight advance upon the 

 one immediately preceding it. It is, nevertheless, a true metamorpho- 

 sis, involving the acquirement of new structures, the atrophy of others, 

 and change of function of still otliers ; the structural ditt'erenco between 

 the young larva and the adult prawn being much greater than in insects 

 with a so-called incomplete metamorphosis, like the Orthoptera. 



I am inclined to think there is error in many of the observations 

 recorded of consecutive stages in the development of the Macroura 

 captured with the hand-net or tow-net. In most of these cases the 

 great difference in form between the specimens warrants the assumption 

 that intervening stages are missing. Consecutive stages which I have 

 actually reared in confinement often do not differ appreciably from one 

 another in size, so that one cannot be sure from the relative size alone 

 of his larva; that links are not wanting in his chain of forms. 



Excepting the statement of Agassiz that Palcemonetes vulgaHs hatches 

 from the e^g as a Curtia* the only notice of the developmental history 

 of this species, with which I am acquainted, is the short description of 

 the first larval stage, by S. I. Smith, already cited. t 



But few observations have been made upon the development of the 

 European species of Palcemon, which is somewhat strange when one 

 considers how common some of them are. 



The first naturalist who published anything concerning the develop- 

 ment of the genus was Rathke, who made some observations in 1833 

 iipon the growth of the embryo within the egg of Palwmon adspers^is 

 Rathke (= P. rectirostris Zaddach, according to Heller), | and Palce- 

 mon squilla.^ These observations are so full of error that they have 

 only an historical interest. Prejudiced by his studies upon the devel- 



* Amer. Jour. Sci. and Arts, 2d Series, Vol. XIII. p. 426. 1852. 



+ Supra, p. 304. 



+ Die Crustaceen des siidlichen Europa, p. 270. 1863. 



§ Ueber die Entwickelung der Decapoden. !Miiller's Archiv, 1836, pp. 187- 192. 

 Zur Morphologie, Reisebemerkungen aus Taurien, pp. 81-93, 179-184, IM. IV. 

 Figs. 1 - 10. 1837. 



