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Show 1882.] PROF. F. J. B E L L O N T H E G E N U S PSOLUS. 649 the test. The differences do not seem to me to be of really generic value, any more than are the distinctions which some have seen between what have been called Psolus and Cuvieria, or Psolus and Lophothuria, no naturalist, so far as I know, having followed Bronn (Classen u. Ordn. i. p. 404) in the use of the term Lepidopsolus. A naturalist need know no other species than P. fabricii and P. phantapus to see what are the kind of claims for generic separation. In the one case there is a heavily-armed test, formed of strong imbricating scales, with only the margin of the foot provided with sucking-feet, and with the tentacles richly branched; in the other there are granulations, less richly branched tentacles, and a median set of sucking-feet1. An investigation of the internal anatomy will not, however, reveal a difference in the part which should especially be affected in the more firmly bodied forms. W e might, that is, expect to find valuable distinctive marks in the grade of development of the Polian vesicle, the size of which in P. fabricii, or any other heavily-armed form, would be easily enough ascribed to the fact that the impossibility of the walls of their body aiding in the propulsion of fluid through the ambulacral canals would require the propelling organ to be of larger size, and doubtless also of greater proportional strength. A priori considerations of this kind are often shown by the dry light of dissection and observation to be as little in consonance with fact as the nature of things allows; and that is certainly the case here : the Polian vesicle of Psolus regalis is proportionally as large as, even if it be not larger than, that of P. fabricii. A fact of this kind does, at the same time, teach us that what is apparently an external difference of great importance may be so as between, say, Psolus and Holothuria, but is not a great one between Psolus and Lophothuria. Such being the case, we have here an example of affinities so peculiar that what very rarely obtains among Echinoderms, at any rate, does seem to be presented here-a relationship that can best be indicated in the language of systematic zoology by making use of subgeneric divisions. While Psolus may be spoken of as a Gasteropodous dendrochiro-tous Holothurian, with a flattened trivium and the bivium without suckers, and invested in a firm covering of calcareous pieces, Psolus (Eupsolus)s. str. will have granular plates, a median row of trivial suckers, and no basal web to the tentacles; Lophothuria large granulated scales or plates, no median row of suckers, and a basal web to the more richly branched tentacles; while Hypopsolus has the scales invested in a thick integument, and the trivial suckers numerously developed. It is possible that future investigation will justify us in associating with these, as another subgenus, Lissothuria2, where "the upper surface of the body is covered with a soft smooth skin, in which are imbedded minute perforated plates." 1 See Verrill, Proc. Bost. Soc. Nat. Hist. x. p. 353. 3 Trans. Conn. Acad. i. p. 322. |