248 A HISTORY OF RECENT CRUSTACEA 



Leacli, was at one time called Anchistia migrator la by 

 Heller. But it differs from Dana's Anchistia by having 

 three flagella instead of only two on the first antennas, and 

 by having the two spines of the carapace one above in- 

 stead of one behind the other. Heller therefore instituted 

 for it in 18G9 the new genus Palcemonetes, which agrees 

 with Anchistia but differs from Palcemon in having: no 

 ' palp ' to the mandibles. The development and larval me- 

 tamorphoses of Palcemonetes various have formed the sub- 

 ject of important studies by Dr. Boas, Mr. W. Faxon, and 

 Dr. Paul Mayer. Its distribution has been summed up by 

 Professor Th. Barrois. The same genus includes also the 

 American fresh-water species, Palcemonetes exilipes, Stimp- 

 son, and Palmmonetes vulgaris (Say), which appears like 

 varians to be almost equally at home in salt water and 

 fresh. It affords an abundant supply of food to many of 

 the fishes on the east coast of the United States, and is in 

 turn itself supported with equal liberality by other animals. 

 Messrs. Verrill and Smith, after speaking of its inhabiting 

 the brackish pools and ditches, even where the water is 

 but little salt, remark that it ' also occurs in immense 

 numbers on the muddy bottoms and among the eel-grass of 

 the estuaries. In the pools there are also myriads of small 

 Entomostraca of many kinds, upon which the prawn and 

 other species feed, while the Entomostraca find an abundance 

 of ciliated Infusoria and other microscopic animals for 

 food.' In.regcrd to Palcemonetes varians an observation, 

 perhaps generally applicable, was made by Hensen and 

 confirmed by P. Mayer, that specimens in confinement will 

 speedily shed their skins if well fed. The same authors 

 showed also by very conclusive experiments that the 

 animals after exuviation introduce fresh otoliths into the 

 ear-cavity of the first antennae. Dr. Mayer supplied 

 crystallised silver, and had the satisfaction of seeing a 

 bright scale of this metal in one ear of his young Palce- 

 monetes. Hensen points out that the introduction of 

 otoliths must as a rule take place while the softness of the 

 new skin permits the narrow opening into the ear-chamber 

 to be stretched to the utmost limit. 



