ME. P. H. CABPENTEE ON THE GENUS ACTINOMETEA. 25 



only two palmars in each secondary arm, although the number of distichal segments in 

 the primary arms is usually three. Another proof, if proof were wanted, that the arms 

 proper of the articulate Criuoids begin from the axillary radial is seen in the fact that 

 whenever there are three segments in a distichal or palmar series, the second of these, 

 which is united to the first by ligament only, always bears a pinnule, while the third, or 

 axillary, is a double segment, i. e. it consists of two primitive segments united by a 

 syzygium. This is in precise accordance with what we find in all the ten-armed 

 Comatulce, even in those forms in which, as in. Act. Solaris, the first and second brachials 

 are united by a syzygium like the second and third radials. In these the second brachial 

 or the epizygial element of the syzygium bears a pinnule, while the next segment is also 

 a double one, and corresponds with the compound third brachial of Ant. rosacea and of 

 the ordinary ten-armed Comatulce. In these last the second brachial is laterally movable 

 upon the first, and bears a pinnule as in Act. Solaris, while the third has a syzygium (/. e. 

 is a double segment). This is exactly what Ave find to be the case in those primary and 

 secondary arms of the multiradiate Comatulce which consist of more than two segments. 



(§ 18) The principal character of the genus Actinometra is, as we have seen in sect. 14, 

 that the mouth is situated excentrically, while the centre of the disk is occupied by the 

 anal tube. The position of the mouth relatively to the radii or ambulacra, however, is 

 not the same in all Actinometrce ; thus in Act. Solaris (PL I. figs. 2, 5) the mouth lies 

 in a radial or ambulacra! plaue, while in Act. WahlbergMi and many other species (PI. I. 

 figs. 3, 4, 6-16) it is interradial or interambulacral. If we place the disk of an ordinary 

 Antedon in such a position that the interradial area containing the anal tube is nearest 

 to us (PI. I. fig. 1), the odd ambulacrum lies in front of the mouth. Let us designate 

 this as ambulacrum or radius A, and the two branches of its groove-trunk corresponding 

 to the two primary arms as A L and A 2 respectively, A,, being that on the left of the 

 mouth. Proceeding round the disk in the direction of the hands of a watch, we may call 

 the other four ambulacra B, C, D, E respectively, and their primary divisions L\ B.. 

 . . . . E x E 2 . The anal area is then bounded by the two postero-lateral ambulacra C, D ; 

 and a plane passing through the mouth and anus, so as to divide the disk into two 

 symmetrical halves, passes along the odd ambulacrum or radius A, in front of the mouth, 

 which may therefore be regarded as radial in position. 



In Act. Solaris the same is the case, as may be seen in Midler's somewhat diagrammatic 

 figure (PL I. fig. 2) 1 , and still better in PL I. fig. 5, which was drawn from a spirit- 

 specimen, and not from a dry one like Midler's figure. Here, as in Antedon, the odd 

 ambulacrum is in front of the mouth, which, although excentric in position, lies in the 

 radial half of a plane which passes through the mouth and anus, so as to divide the disk 

 into two symmetrical halves. The same is the case in a new Actinometra from the 



1 It is not usual to meet with specimens of Actinometra in which the branches of the ambulacral grooves are 

 distributed with such symmetry as is represented in Muller's diagrams (PL I. figs. 2-4) and in PI. II. fig. 1. Thus, 

 for example, Midler's figure of Act. Solaris (PL 1. fig. 2) is remarkably regular, much more so than that repre: en1 ! 

 in fig. 5; and I have examined other specimens with more than 20 arms and a radial mouth, in which the regularity 

 is by no means so distinct as in PI. II. fig. 1. A great range of variation in this respect is seen in PL I. figs. 6-16. 

 which represent the disks of eleven different individuals of Act. jpolymorpha, no two of which are alike ; the position 

 of the mouth, however, is constant in all individuals of the same species. 



SECOND SERIES. — ZOOLOGY, VOL. II. 4 



