2G MK. r. H. CAEPENTER ON THE GENUS ACTINOilETEA. 



Pliilii^piues (PI. II. fig. 1) with 23 arms. In both these species the dividing plane passes 

 in front of the mouth between the two primary divisions, Ai A, of the odd anterior 

 ambulacrum A, while behind the mouth it is interradial, and separates the two postero- 

 lateral ambulacra C, D. 



In Act. JFahlbevghil (PI. I. fig, 3), Act. muUiradiaia (fig. 4), and Act. iwlymoi-plia 

 (figs. 6-lG) the case is dilferent. If, as in Antedou, we designate the two ambulacra 

 bounding the anal area as C and D respectively, we find that the latter is the odd 

 ambulacrum, and that a plane cutting the mouth and anus is radial behind the mouth, 

 in front of which it passes between the two ambulacra A and B ; so that if the centre of 

 the disk be regarded as the centre of radiation, the mouth lies in an interradial or inter- 

 ambulacral plane. This is clearly seen when we turn to the dorsal side of the disk, in 

 which the radii converge to a central point, and not to an excentric one, like the ambu- 

 lacra of the ventral side. Thus in PI. II. figs. 9-11, the position of the mouth relatively 

 to the radii is indicated by a X , which in each case is between the two anterior radii 

 A and B, or interradial. 



So far as my experience goes, this type of Actinometra, in which the mouth is inter- 

 radial and the odd ambulacrum lies behind it, is slightly more common than the simpler 

 tyj)e, in which the mouth is radial and the odd amlnilacrum anterior, as in Antedou '. 



(§ 19) We are now in a position to investigate which of the numerous species of 

 Comatulcs described by Midler belong to the type of Antedou, and which to Actinometra, 

 and into what groups the latter may be divided according to the principles of classifica- 

 tion discussed in the last four sections. Miiller's specific descriptions afford very little 

 information in this respect ; for though he says that the mouth, in some instances, is 

 excentric, and that in others the oral j)innules have a terminal comb, he does not always 

 do so ; and he makes no use Avhatever of these two characters in his distinction between 

 Alccto and Actinometra. In some cases he simply designates a species as Comalula, 

 without attemping to name it more exactly. This is often, no doubt, simply due to the 

 fact that, in the specimens which he examined, the arms were so closed over the disk that 

 he was unable to investigate the distribution of the ambulacra. This, however, is not the 

 case in the C. trichoptera of the Paris Museum, the disk of wliich can be readily ex- 

 amined ; and I believe that Miiller did not define this species more precisely because he 

 was unable to decide whether it should be referred to Alecto or to Actinometra ; for in 

 one of the two Paris specimens five groove-trunks start from the excentric peristome, 

 while in the other there are only four. This example alone suffices to show the unsatis- 

 factory nature of the only distinctive character established by Miiller between his genera 

 Alecto and Actinometra, The Paris Museum^ contains a very large majority of the 



' Since the above -was written, I have examined three large Con!rt<((7a-collcctions : — (1) that of the ' CTiallengcr ; ' 

 (2) that made by I'rof. (jemper in the I'hilippines ; and (3), thanks to the kindueis of Dr. Giiuther, that in the 

 British iluseum. 



I hare been able to determine tlio position of the mouth in 80 species of Actlaometra. In 4.'5 of these it is inter- 

 radial, as in Act. pohjmofj^iha ; whUe in the remainig 35 it is radial, as in Antedon and in Act. Solaris. 



- I would here express my most hearty thanks to Mons. Edmond Perrier, Assistant- Naturalist at the Museum of 

 Natural History, Jardin des Plantes, who has charge of the Echinodcrm collection, and also to his two Assistants, for 

 the kindness which they showed me during my stay in Paris, aud for the readiness with which they afibrdcd me 

 every possible facility in the prosecution of my work. 



