ON MYSIS. 19 



sary with nutiltioii to the continuance of life and the 

 evolution of the foetus, as the subgelatinous secretion 

 iippcars to exclude the direct influence of the ocean upon 

 the respiratory organ, which moreover does not appear 

 to be developed until the moment prior to their exclu- 

 sion from the pouch, this circumstance, taken in conjunction 

 with the suspicions of some Physiologists as to the oxi- 

 gcnation of the foetal blood, may lead to such further 

 observations as may tend to throw some new light upon 

 this still obscure function in the foetus. 



To the Philosophical Naturalist, who delights to trace 

 the changes wliicli parts are made to assume in figure and 

 use, in contiguous groups of beings, so as to fit them for 

 ditFerent modes of life, the Opossum Shrimp must prove 

 Iiighly interesting, for independent of the peculiar structure 

 or modification of its feet before noticed, and which renders 

 it essentially natatory, it points out to us in the clearest 

 manner possible, by the changed appearance of its three 

 anterior pairs* of feet, that wliat have been considered in 

 the Crabs and Lobsters, &c. (Decapoda) as three pair of 

 auxiliary jaws or Pedi-maxilla?, are no other than the 

 above members in a disguise which seems to appropriate 

 them in a decided manner to the service of the mouth ; for 

 in tlie Decapoda there are (as the name of the family 

 implies) but five pair of feet, and three pair of auxiliary 

 jaws, which jaws being added to the front of the series of 

 feet in the Opossum Shrimps, encreases their number to 

 eight pairs, of which the sixtli, reckoning upwards, are 

 not to be distinguislied in structure from those posterior 

 to them. 



The males in this Genus appear to be much less nume- 

 rous than the females, and dilfer principally in being 

 smaller, and in the substitution of a peculiar organ 

 between the last pairs of feet (Plate I. fig. 13, 14.) instead 



* ']'he peculiar stntctiire of tlio linili"5, l)ciiie sululividtxl into two, rcuders 

 ucccssary this uiiauiliorizcd ajpliccitiwi) of fairs. 



