444! Ml?, p. H. CAKPEIS'TEK OS THE GENUS ACTINOMETRA. 



Antedon: he has only met with this last in A. Solaris and its 

 varieties, in A. jimhriata, and in three new species oi Actinometra 

 from the "West Indies ; all the other species known to him belong, 

 like A. multiradiata and A. polymorpha, to the type in which the 

 mouth is interradial and the odd ambulacrum posterior. 



The median line of the ventral perisom of all the arms of 

 Antedon is occupied by an open ambulacral groove bounded by an 

 elevated fold of perisom, the edge of which is not straight, but cut 

 out into a series of minute valvules, the crescentic or respiratory 

 leaves. At the base of each leaf, and to some extent protected by 

 it, is a group of three tentacles, one of which, the more distal, is 

 larger than the other two. This trifid group of tentacles and the 

 cavity of the respiratory leaf in connexion with them, are in com- 

 rnuuication with the cavity of the radial water- vessel by a common 

 lateral branch : the tentacular groups alternate on the opposite 

 sides of the ambulacral groove, from the base to the tip of each 

 arm, and are distributed in the same manner at the sides of the 

 ambulacra of the disk. The floor of each groove consists of a layer 

 of ciliated epithelium, beneath which lie the radial water- vascular 

 and blood- vascular trunks, but separated from it by a fibrillar 

 structure (the subepithelial band), to which a nervous character 

 has been attributed by the author and by all the other observers 

 'who have described it. 



In Act. polymorpha and in many other Actinometrce the above 

 description only applies to the more anterior arms ; for the arms 

 of the posterior radii are usually entirely devoid of tentacles, and 

 in many of them the ventral perisom not only exhibits no am- 

 bulacral groove, but is convex as in the oral pinnules of Antedon. 



In neither of the two large aboral groove-trunks which form a 

 horseshoe-shaped curve enclosing the anal area oi Act. polymorpha, 

 are the tentacular groups so well developed as in the two anterior 

 ambulacra. The bases of these trunks, where they start from the 

 peristom, are of the usual character ; but sooner or later the ten- 

 tacles become more and more insignificant, and finally disappear 

 altogether, while the position of the crescentic leaves is only in- 

 dicated by a very faint wavy line at the edge of each groove. In 

 small specimens with but few arms, the grooves of the posterior 

 and posterolateral arms may remain in this condition; but in 

 larger specimens with many arms all trace of the respiratory leaves 

 disappears, and the two edges of the groove gradually approach 

 one another and finally unite, so that the ventral surface of tlie 



