Description of a Twelvc-armcd Comatula. 



181 



armed one in question, and the small size of the two arms at 

 B suggests that they may have been regenerated.^ The fol- 

 lowing measurements will serve to show the great size of 

 the twelve-armed Antedon : 



Greatest diameter of disk in line through mouth and anal cone, 0*62 inches. 

 Diameter in line perpendicular to the last, . . 0*56 ,, 



Greatest diameter of mouth. 

 Least diameter of mouth, 



Height of anal cone, 



Width of anal cone at base. 



Length of oral pinnules, 



Width of brachial plates at base of arms, 



0-09 

 0-06 

 0-14 

 0-14 

 about 0-62 

 0-08 



There were about forty well-developed dorsal cirri, and 

 about a dozen more rudimentary ones. All the arms are of 

 about the same diameter near the base (this is not sufficiently 

 clear from the figure), and there is no appearance of re- 

 generation having played any part in the production of the 

 extra ones. It appears to me that the presence of two addi- 

 tional arms in some degree accounts for the large size of the 

 animal, for it thereby secures a distinct advantage, having 

 twelve arms in place of ten wherewith to catch its food, and 

 twelve ambulacral grooves to convey the food, when caught, 

 to the mouth. 



Considering the mouth as anterior, the anus posterior, a,nd 

 right and left sides accordingly, as in the figures, the arrange- 

 ment of the twelve arms is seen to be as follows. (In the 

 figures the arms are numbered consecutively. Starting at the 

 side opposite to the anus, i.e., anterior, the arms of the right 

 side are E. 1, E. 2, E. 3 <x, E. 3 h, E. 4, and E. 5; those of the 

 left side are L. 1, L. 2, L. 3, L. 4 a, L. 4 I, and L. 5— E. 5 

 and L. 5 being immediately right and left of the anal cone.) 



The figure (Fig. 2) shows that the first and second arms of 

 the right side arise in a perfectly normal manner ; the third 

 arm on this side, however, divides into two — E. 3 a and 

 E. 3 h — the second brachial plate (Br. 2) acting precisely 



1 Since going to press. Professor F. J. Bell has kindly pointed out to me 

 that the only specimen obtained of his Actinometra coppingeri {vide "Zool. 

 Coll. H.M.S. 'Alert,'" Brit. Mus., 1884, p. 168, PI. xvi., fig. b) possesses 

 twelve arms. He says {loc. ciL), "The specimen under examination has 

 12 arms, but the normal number is probably 10." He also informs me that 

 it is a good-sized specimen. 



