MR. P. H. CARPENTER ON THE GENUS ACTINOMETEA. 41 



The arms of Act. polymorpha may thus be roughly classified as follows : — 



Anterior, on radii A and B, 120-150 segments. Pinnules increasing in length to the terminal ones, 

 which are very long and slender. Tentacnliferous. 



Anterolateral, on C t and E s , 100-120 segments. Terminal pinnules long and slender. Tenta- 

 cnliferous. 



Posterolateral, on C 2 and E,, 80-100 segments. Terminal pinnules stout, and rather longer than the 

 median ones. Usually have " sense-organs " and narrow ambnlacral grooves, but are non-tentaculiferous. 



Posterior, on radius D, 60-80 segments. Terminal pinnules stout, but shorter than median ones. 

 Sense-organs. Usually no grooves. Non-tentaculiferous. 



Another difference between the anterior and posterior arms is that the genital glands 

 in 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 

 more than half that of an anterior arm, but they also attain a very much greater size ; 

 the basal and median pinnules of an anterior arm being very much less swollen than 

 the corresponding pinnules of a posterior arm. A similar inequality in the development 

 of the genital glands has been noticed by Alex. Agassiz 1 as occurring in the Echini. This 

 difference in length in the anterior and posterior arms of. Act. polymorphs, and in the 

 character of their terminal pinnules, seems to be to a certain extent dependent upon 

 the condition of the respiratory apparatus occupying their ventral surface. When this 

 is well developed the arm seems to have the power of indefinite growth ; for in the 

 single specimen (PL I. tig. 16) in which all the thirty-three arms were normal and ten- 

 taculiferous as in Antedon, there was no very appreciable difference in the lengths of 

 the anterior and posterior arms-. The shape of the terminal pinnules, however, was of 

 a slightly different character in the two cases, though the development of the genital 

 glands was about the same ; and we have just seen that those arms are the shortest in 

 which the ambulacral groove entirely closes, and the water-vessel is reduced to a simple 

 tube without any lateral tentacular branches, while it is in these arms only that any 

 definite mode of termination is known. This may occur before half the number 

 of segments have been developed which are commonly met with in an anterior ten- 

 tacnliferous arm. 



(§ 27) The ventral surface of some of my specimens of Act. polymorpha is marked by 

 small calcareous concretions, somewhat resembling the " blumenartige Knotchen mit 

 mehreren blattartigen Fortsatzen " described by Midler 3 in the Vienna specimen of 

 Act. Solaris. When present, they are usually scattered around the peristome, and 



1 ' Revision of the Echini," part iv. pp. 680, 681. 



2 Not only are the arms of different, lengths in the ' Challenger ' species of Actinometra, which have ungrooved 

 hinder arms, but there are three species in which the anterior arms are longest, although all, anterior and posterior 

 alike, are grooved and bear tentacles. In another species the arms are all grooved and all equal in length, but the 

 distribution of the syzygia is quite different in the anterior and posterior arms. 



:i • Gattung C'omatula,' p. !"_'. 



SKCON'U SERIES. — ZOOLOGY, VOL. LI. b" 



