1884.] PROF. F. J. DELL ON THE GENUS AMPHIC\CLUS. 255 



grouped under the three subfamilies of Stichopoda, Gastropoda, and 

 Sporadipoda, according as the ambulacral suckers were set in definite 

 rows, and the interradii were altogether or almost completely devoid 

 of suckers (e. g. Cucumaria), were confined to the trivium (" ventral 

 surface ") (e. g Psolus), or were scattered more or less regularly 

 over the whole body, as in Thyone. 



Among the Sporadipoda, Thyone and Stereoderma alone had ten 

 tentacles only ; and, till the time of Ludwig's institution of Pseudo- 

 cucumis and Actinocucumis, all Stichopods were thought to have ten 

 tentacles or to be " decachirote." 



The recent researches of Von Marenzeller have resulted in an 

 emendation of tbe generic characters of Colochiru.i \ and have in 

 principle removed it from the Stichopoda to the Sporadipoda, so 

 that in place of saying with Semper " Die Fiisschen der Bauchseite 

 stehen in '6 deutlich von einander getrennten Reihen," we now say, 

 with Marenzeller, " Die Ambulacralt'usschen der Bauchseite stehen 

 entweder in drei deutlicheu Reihen oder nahezu regellos." While 

 Von Marenzeller has demonstrated the inconstancy of tbe Stichopod 

 arrangement in one of the Stichopoda, it has been my fortune to 

 show 2 that the Sporadipod disposition of the ambulacral feet in 

 Stereoderma is, in S. murrayi, carried further than it is in S. uni- 

 semita, the only species of the genus that was known in 1868. 



We are then led to the conclusion that the disposition of the 

 ambulacral suckers offers a less certain basis for arrangement than 

 was supposed some years since. It might, indeed, well have been 

 thought that as the Holothurian got further and further away from 

 the parent stock which remained under the domination of the 

 pentamerous disposition of parts, it would, as it began to develop 

 more than five pairs of tentacles, have its sucker-feet developed in 

 the interradial as well as the radial parts of the body. Such a 

 theoretical consideration would find support in the fact that some 

 forms as they grow older lose a stichopodous and acquire a sporadi- 

 podous arrangement of the sucker-feet ; while a not unimportant 

 consideration for the systematist is the variability of this character. 



With regard to the former, however, opposing evidence is offered 

 by the case of Amphicyclus, where, with in all 24 tentacles, we have 

 the stichopod arrangement completely retained ; and by the genus 

 Actinocucumis, made known to us by H. Ludwig, where there is a 

 stichopod arrangement, and from 18 to 20 tentacles. 



As to the latter consideration, I should like to speak with diffi- 

 dence till I have a better acquaintance than I have now with the 

 species of the genus Cucumaria. As a matter of fact, however, 

 systematists do, at this moment, unite under that head (a) stichopods 

 with ten equal tentacles, (/3) stichopods with ten tentacles of which 

 two are smaller than the rest, and (y) forms with eight large and two 

 smaller tentacles, and some sucker-feet scattered in the iuterambu- 

 lacra. Semper has hinted at the advantage of separating the last 

 from the rest and forming for them a new generic group ; but he 



1 Verh. zaol.-bot. Ges. Wieu, 1881, p. 129. 

 8 P.Z.S. 1883, p. 61. 



18* 



