450 MR. p. n. CARPENXEE ON THE &ENUS ACTINOMETRA. 



The same variation occurs in Actinometra solans, in wMdi spe- 

 cies the number of arms is limited to 10 : they may be all tenta- 

 culiferous ; or from one to four of the posterior arms may have no 

 tentacular apparatus. This abnormal condition does not seem, 

 however, to be very common ; for out of all the Actinometra in the 

 Paris collection the author found but one in which he could say 

 with any certainty, without cutting sections, that some of the pos- 

 terior arms were non-tentaculiferous ; and this was a large many- 

 armed specimen of A. Bennetti, Miiller. 



The condition of the ambulacral groove and of the tentacular ap- 

 paratus is not the only point in which the anterior or oral arms of 

 Actinometra may differ from the posterior or aboral arms. The 

 former taper very slowly, contain far more segments, and are 

 much longer than the latter, while the form of their terminal 

 portions, and of the pinnules which these bear, is altogether 

 different. 



In A. polymorpha the centre of the dorsal half of each of the 

 segments of the terminal pinnules of the posterior arms is often 

 occupied by a dark brown ovoid body of a peculiar cellular nature, 

 which the author has reasons for believing to be a sense-organ. 

 These bodies may also, though rarely, occur in one or more of the 

 anterior tentaculiferous arms ; but they do not exist in all the 

 specimens of ^. polymorplia which the author has examined, for 

 in 7 out of 12 specimens they are entirely wanting. 



The arms of A. polymorplia may therefore be roughly classified 

 as follows : — 



1. Anterior. — 120-150 segments : pinnules increasing in length 

 to the terminal ones, which are very long and slender. Tentacu- 

 liferous. 



2. Anterolateral. — Also tentaculiferous : 100-120 segments : 

 terminal pinnules long and slender. 



3. Posterolateral. — 80-100 segments : terminal pinnules stout, 

 but rather longer than the median ones. Ventral perisom with 

 narrow ambulacral grooves, but non-tentaculiferous. 



Posterior. — Only 60-80 segments : terminal pinnules stout, 

 but decreasing slightly in length from the middle of the arm 

 onwards. No ambulacral grooves nor tentaculiferous apparatus. 



Another difference between the anterior and posterior arms is 

 that the genital glands of the latter are far more developed than 

 in the former. Not only is their number greater, although the 

 total number of pinnules on a posterior arm may not be much 



