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Show 256 PROF. F. J. BELL ON THE GENUS AMPHICYCLUS. [May 6, has not done more than give a hint, which has not till yet been acted upon. Any statement as to the phylogeny of the Holothurians must be made with the greatest caution, for this reason, if for no other, that our knowledge of the palaeontological history of the class is almost nil \ If, therefore, in what follows there appears to be anything like dogmatic statements, the student will remember that such a mode of presentation is often convenient on the score of brevity. In distinguishing the two families of pedate Pneumonophorous Holothurians, the form of the tentacle is an important factor; in distinguishing the genera of one of these suborders is it not probable that the number and mode of disposition of the tentacles may be well taken into consideration ? A m o n g the Dendrochirotae, some species of Cucumaria and Psolus alone retain the primitive arrangement of five pairs of equal tentacles: Psolus early left the common stock and is, really, a form which is only essentially modified in the gastropod or trivial disposition of its tube-feet. Some of the Cucumariae acquired the differentiation of a pair of tentacles smaller than the rest; thence branched off Ocnus and Colochirus, in which the suckers tended to be confined to the trivium, and in which a large deposit of calcareous bodies was still retained in the integument; of Colochirus, Stereoderma is a more specialized form. Thyone, with four pairs of longer and one of smaller tentacles, has a sporadipod arrangement of the ambulacral feet; in Thyonidium five pairs of smaller tentacles are developed in addition; in Orcula and Phyllophorus a sporadipod arrangement of suckers is associated with an irregular disposition of the feet; while in Pseudocucumis ten pairs of tentacles, in Amphicyclus twelve pairs regularly arranged, lead to Actinocucumis with its irregularly disposed tentacles but its stichopod feet. If, therefore, w e retain the Semperian method of classification, we widely separate Psolus, with its primitively arranged tentacles, from Cucumaria ; widely separate Orcula and Phyllophorus, with irregular and variable tentacles, from Actinocucumis; and place with the Stichopoda Colochirus, in which a sporadipod arrangement is perhaps not less rare than is a stichopod disposition in Thyonidium, which belongs to the Sporadipoda. By writing T for the large primary tentacles, T' for the secondary (more than 10) large tentacles, and t for the small tentacles, we can at a glance see the relations of the genera in this particular. Cucumaria . 10 Tor 8T + 2*. Psolus 10 T. Colochirus 8T + 2*. Stereoderma ST-\-2t. Ocnus 8T + 2*. 1 Consult the single page (pp. 559-60) devoted to Holothurians in Zittel's valuable ' Handbuch.' |