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 limbs, the abdominal appendages 
make their appearance, first the posterior pair (which develop within 
the caudal plate or telson), then the anterior pairs. 
The different forms which the individual assumes during its develop- 
ment are in this case the result of gradual growth, cach 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 others ; the structural difference 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 ۵ 
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 larve that links are not wanting in his chain of forms. 
Excepting the statement of Agassiz that Palcmonetes vulgaris hatches 
from the egg as a Cuma,* 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.+ 
But few observations have been made upon the development of the 
European species of Palæmon, 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 
upon the growth of the embryo within the egg of Palemon adspersus 
Rathke (= P. rectirostris Zaddach, according to Heller), and Palæ- 
mon squilla.§ These observations are so full of error that they have 
only an historical interest. Prejudiced by his studies upon tke devel- 
* Amer, Jour. Sei, and Arts, 2d Series, Vol. XIII. p. 426. 1852. 
+ Supra, p. 304. 
t Die Crustaceen des südlichen Europa, p. 270. 1863. 
$ Ueber die Entwickelung der Decapoden. Müller's Archiv, 1836, pp. 187 = 192. 
Zur Morphologie, Reisebomerkungen aus Taurien, pp. 81-93, 179-184, Pl. IV. 
Figs. 1-10. 1837. 
