Iviii 



INTRODUCTION, 



Fig. j. 



of Galaihea and Palinnriis, excepting that on the superior 

 rings of the tail in the latter are situated four pairs of 

 appendages (fig. j). Upon this point Mr. Couch has the 



following sensible remarks. 

 " There is not certainly the 

 same difference of configura- 

 tion between the young and 

 adult condition of these (the 

 Macroura), as is found to be 

 the case with the short-tailed 

 crabs, simply from the circumstance of the tail being- 

 extended in both states, and the claws also show a 

 nearer ajDproach to each other. But this similarity is 

 more apparent than real, for the physiological difference 



is nearly as wide in one case as in the other 



The form of the shield and the body generally, the 

 sessile character of the eyes, and the long and slender 

 filaments on the tail in PalimirKS, undergo an entire 

 change in the transformation. The shield and body 

 become more depressed and elongated, the eyes be- 

 come elevated on stout footstalks," &c. The sessile 

 character of the eyes in the early stage of all the Po- 

 iJopJtthalma hitherto examined is a very remarkable and 

 important character. 



The changes in the smaller decapod Macroura, repre- 

 sented by the genus Palemori, were first examined by 

 Mr. Thompson, and formed the subject of a second 

 paper read before the Royal Society in 1836. This paper, 

 as well as that on Carcinus before referred to, appears 

 not to have been known to the late Capt. du Cane, who 

 having amused the hours of a long illness by a number 

 of interesting investigations on subjects of Natural His- 



