40 ME. P. H. CARPENTER ON THE GENUS ACTINOMETRA. 



I have never been able to ascertain what is the precise mode of termination of these 

 anterior arms ; even when the arm ends in such a manner that there is no reason to 

 suppose that its terminal segments have been broken off, its few last pinnules appear 

 simply as immature, and the last pair are separated by a delicate prolongation of the 

 arm-stem, on which no pinnules have been as yet developed. Dr. Carpenter : has found 

 the same " growing-points " at the ends of the arms of Ant. rosacea, all of which are of 

 the same character as the oral tentaculiferous arms of Actinometra ; and he was never 

 able satisfactorily to determine the normal mode of termination of the arms. 



With the posterior arms of Actinometra, however, the case is different. From the 

 twenty-fifth segment onwards they taper very rapidly, and instead of reaching a length 

 of 145 millims., as the anterior arms with some 150 segments may do, they have only 

 some 80 segments, and rarely attain a greater length than G0-70 millims. 



At the same time their terminal pinnules are little, if at all, longer than those of the 

 middle portion of the arm (PI. II. figs. 5, 6) ; and the centre of the dorsal half of each 

 of- their segments is occupied by a dark-brown egg-shaped body, of a peculiar cellular 

 nature, which I have reasons for believing to be a sense-organ 2 (PI. II. fig. 6, o.b). 

 These bodies commence to appear in the pinnules at about the beginning of the second 

 third of the length of the posterior arms, and are continued to their extremities. The 

 pinnules of the last few segments decrease very slowly in size ; and the arm ends in an 

 axillary segment which bears two pinnules of the ordinary character, each provided with 

 the brown ovoid bodies or " sense-organs" (PL II. fig. 6, o.b). 



These bodies, which may occur, though but rarely, on one or more of the anterior 

 tentaculiferous arms, do not exist in all the specimens of Act. polymorpha which I have 

 examined. In three out of my eight specimens of the type they are entirely wanting ; 

 and they are also absent in all the single specimens of the four varietal forms which 

 I have investigated. I have also failed to find them in the non-tentaculiferous arms of 

 Act. Solaris*. 



Between these two kinds of arms, the long anterior ones on the radii A, B, with a 

 wide ambulacral groove and a well-developed respiratory apparatus, and the short 

 posterior ones of the radius D with a closed groove and no external respiratory appa- 

 ratus, all possible forms of transition may occur. As a general rule, more or fewer of 

 the antero-lateral arms, d and E 2 , are tentaculiferous ; but they never reach such a great 

 length as the anterior arms, and their terminal pinnules are by no means so long and 

 slender. At the same time the postero-lateral arms, C 3 and E l9 although generally non- 

 tentaculiferous, have, except in rare cases, a more or less open groove for the greater part 

 of their length, which, while greater than that of the posterior arms of the radius D, 

 is less than that of the antero-lateral arms of C L and E 2 ; and their pinnules increase 

 slightly in length from the middle till near the end of the arm. 



1 Phil. Trans. I860, p. 723, plate xxxviii. fig. 4. = Journ. Anat. & Phys. vols. x. xi. Incc. citt. 



3 Sense organs occur in two of the ' Challenger' species — one from Banda (which is probably the young of Act. 

 jydlymorpha), and one (a new species) from the Admiralty Islands. In both cases they are limited to the hinder arms, 

 some of which are grooved and others not. 



