322 MR. WESTWOOD ON THE SUPPOSED EXISTENCE 



the latter of which is accompanied by four plates. I \^dll therefore content myself 

 with referring the student to these accessible sources, without attempting to give 

 even an outline of the elaborate investigations in question. 



Latreille, speaking of the young of the Cray-fish, says : " Les jeunes ecrevisses, 

 tr^s moUes au moment de leur naissance, et toute-a-fait semblables a leurs m^res, se 

 refugient sous leur queue, et y restent pendant plusieurs jours et jusqu'a ce que les 

 parties de leurs corps soient raffermies *." 



Mr. Thompson himself, in the genus Mysis, has clearly shown that these animals, 

 which he has proved to be most intimately allied to the Decapod Macrowra, undergo a 

 series of changes, which he states " cannot be considered as metamorphoses, but 

 simply a gradual development of parts -}-." 



The above appear to be all the direct observations hitherto made upon the Podoph- 

 thalma (with the exception of those subsequently detailed from my own researches) ; 

 but amongst the sessile-eyed Malacostraca we have more numerous observations. 



Of these, as in the former, the researches of Rathke again stand foremost ; since, 

 in a series of memoirs, very recently published, upon the development of the ova and 

 embryos of various animals, we find the common Asellus aquaticus to have been in- 

 vestigated by him, with the result that no material alteration takes place in the form 

 of the animal %. 



In the Annales des Sciences Naturelles for December 1833, is published a valuable 

 report, by M. Isidore Geoffroy St. Hilaire, upon a memoir of M. H. Milne 

 Edwards, entitled " Observations sur les changemens de forme que les Crus- 

 tac^s ^prouvent dans le jeune age." Passing over the more generalized views de- 

 duced by M. St. Hilaire from the facts noticed by M. Edwards, I shall merely state 

 the latter. The genus Cymothoa, and some other Isopodous genera, afforded to 

 M. Edwards an easy opportunity of examining the development of the eggs and the 

 structure of the young, in consequence of their being inclosed within the large sub- 

 thoracic pouch. Hence he was enabled to ascertain that some organs which are fully 

 developed in the adult animal, are either rudimental or absolutely wanting in the early 

 state : thus, in the latter the animal has only six thoracic segments, and six pairs of 

 legs, although when adult it has seven segments and seven pairs of legs. On the con- 

 trary, other organs, which are fully developed in the young, become rudimental in 

 the adult state : thus, in the former we find a large head, furnished with two large 

 oval black eyes, and the abdominal segments nearly as large as the thoracic ones ; 

 whilst in the adult state the head is extremely small, the eyes are not externally 

 visible, and the abdominal segments are very short and linear. M. Edwards has also 

 made similar observations upon many other genera, especially upon Anilocra, in 

 which a pair of legs is also developed after birth (the same likewise takes place in 



* Rfegne Animal, torn. iv. p. 90. 2nd edit. f Zoological Researches, p. 16. 



X Abhandlungen zur Bildungs ant Entwickelung-geschichte des MenschenundderThiere.4to. 2 parts. 1832, 

 1833. 



