328 PERCY SLADEN TRUST EXPEDITION 



Desmocaris and the Palsemoninse are active, free-living prawns, mostly inhabiting 

 fresh or brackish vs^ater. The Pontoniinae, on the other hand, are marine, and most 

 of them lead a hidden and sluggish life, depending for shelter upon other animals, 

 such as corals, bivalves, echinoderms, and ascidians. Are v^^e to consider that the 

 non-primitive characters which differentiate this group have any connection with their 

 mode of life 1 Clearly, since not all Pontoniinae lead a sluggish existence, it is not 

 possible to regard the diagnostic characters of the subfamily as direct adaptations 

 to such existence. In any case it would be hard to see this significance in the 

 armature of the telson, or in the structure of the mandible, which is no doubt con- 

 nected with some peculiarity, either in the food or in the mode of feeding, shared 

 by the Palsemoninse. It is, indeed, somewhat remarkable to find that the mandible 

 of the most advanced commensals of the subfamily does not difiPer essentially from 

 those of its most primitive members. But M. SoUaud is quite possibly right in con- 

 sidering* that the reduction of the gill series, though it cannot have been caused by 

 the change to a less active life, has tended to bring about such a change. If, how- 

 ever, . the characters of the Pontoniinse as a whole cannot be regarded as direct 

 adaptations to a sheltered life, there is within the group a long series of such 

 modifications, exhibited by almost every organ of the body, and often traceable from 

 genus to genus in a striking manner. 



The History of the Pontoniinse. 



The subfamily Pontoniinse comprises fifteen genera, described at various dates 

 from 1829 to the present day. The first of these to be established was Pontonia, 

 founded by Latreille in 1829t, for a species, commensal with bivalves, which had been 

 named by Petagna Astacus tyrrhenus (Plate 57, fig. 29). By 1837, when H. Milne- 

 Edwards published his Histoire Naturelle des Crustaces, the number of species assigned 

 to this genus had risen to four. Roux in 1831 gave the name Pelias to two prawns 

 of more active habit which are now known as Periclimenes scriptus and P. ame- 

 thysteus, Pelias being preoccupied as a generic name. Periclimenes was given its 

 present name in 1844, by Costa, who at the same time founded Typton for a species 

 living in sponges. In 1851, Peters established Conchodytes for two very specialized 

 species related to Pontonia. Hitherto isolated genera had been founded, but in 1852 

 the first steps were taken towards recognizing the unity of a group of forms which 

 was to become the nucleus of the present subfamily. In that year Dana, reporting 

 on the Crustacea of the United States exploring expedition, separated from Pontonia 

 certain coral-haunting species which he erected into the genera Oedipus and Harpilius. 

 Conchodytes he did not recognize. At the same time he described as allied to these 

 a genus Anchistia, which has subsequently proved to be the same as Costa's Peri- 

 climenes, and indicated in a footnote that the true position of Typton was in this 

 neighbourhood. The name Oedijms was already in use, and Stimpson accordingly 

 changed it in 1860 to Coralliocaris. Palcemonella, also founded by Dana, was placed 

 by him between Anchistia and Palcsmon, and Stimpson, in founding Urocaris, placed 

 * C. R. cli. p. 1158. t References will be found below. 



