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 palseontological 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 ? 



Among the Dendrochirotse, 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 dis- 

 position of its tube-feet. Some of the Cucumarise 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 

 hodies was still retained in the integument ; of Colochirus, Stereo- 

 derma 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 ; iu Orcula and 

 Phyllojihorus 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, we 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 per- 

 haps 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 8T+ 2t. 



Ocnus 8T + 2*. 



1 Consult the single page (pp. 559-60) devoted to Holothurians in Zittel's 

 valuable ' Handbuch.' 



