30 MR. P. H. CAEPENTEE ON THE GENUS ACTINOMETEA. 



does not occupy a central or subcentral position on the ventral surface of the visceral 

 mass as it does in Antedon, but is placed more or less excentrically, and may be some- 

 times almost marginal (PL I. fig. 11). It occupies the centre of the peristome, P, and is 

 bounded by two lips, a large anterior and a smaller posterior one ; so that its opening is 

 very inconspicuous, and usually so much extended in a direction transverse to the 

 antero-posterior diameter of the disk, that it presents the appearance of a simple slit, as 

 is well seen in PL II. fig. 2. 



The circumoral portion of the peristome, or the peristome proper, is a more or less oval 

 depression in the ventral perisome of the disk, which completely surrounds the oral 

 opening, and gives origin to the ambulacral grooves or, more shortly, the ambulacra. 

 Beneath this depression lies the water- vascular ring which gives off a trunk imder 

 each of the ambulacra radiating from it. The number and distribution of these are 

 very variable, as is seen in PL I. figs. 0-16. This principally depends upon the 

 way in which the ambulacra divide, so as to give rise to the groove-trunks corresponding 

 to the ten primary arms. As a general rule, the two ambulacra corresponding to the 

 radii T) and E unite into one large posterior trunk, from which the branches are dis- 

 tributed to the various arms into which these radii divide (PL I. figs. 8-10, 12-10). In 

 other cases the left lateral ambulacrum, E, leaves the peristome alone (figs. 6, 11) ; while 

 in others it is partially united with the posterior ambulacrum. D, its anterior division, 

 E 2 , leaving the peristome by a separate trunk, while its posterior division, E„ unites with 

 the posterior ambulacrum (fig. 7). 



As a general rule, the right lateral ambulacrum, C, leaves the peristome alone, and 

 supplies the arms of the corresponding radius ; but in figs. 9 & 15 it is seen to unite with 

 the posterior division, B 2 , of the right anterior ambulacrum, B. 



The mode of division of the two anterior ambulacra is excessively variable. As a 

 general rule there are no principal trunks corresponding to the two radii A and B, and 

 the primary divisions, A 13 A,, B^ B,, start directly from the peristome. In the specimens 

 with but few arms, however, each pair may be united for a longer or shorter distance 

 (PL I. figs. 0, 7), as in Antedon (fig. 1). Not unfrequently the posterior divisions 

 A l5 B 2 , of these two anterior ambulacra unite for a longer or shorter distance with the 

 two large ahoral groove-trunks, to form an open horseshoe-shaped curve bounding the 

 anal area (figs. 12, 15, 10). The position of the anal tube in this ai'ea, and also with 

 regard to the whole surface of the disk, varies somewhat with the position of the mouth ; 

 it is rarely, if ever, absolutely central. Its appearance differs very much according 

 as it is full or empty : sometimes its aperture is so completely closed as to be scarcely 

 discernible, though the tube below is widely distended ; and sometimes the aperture is 

 patent with its edges everted and crenate, and the tube leading to it quite shrunk and 

 flaccid (PL II. fig. 2). 



(§ 22) In Antedon the median line of the ventral perisome of all the arms is occupied 

 by an ambulacral groove, with a floor of ciliated epithelium. This groove extends also 

 on to all the pinnules, wdth the exception of those borne by the second distichals and 

 second palmars, &c. (when present), and by the lowest brachial segments. Beneath it 

 lie the radial water-vascular and blood-vascular trunks, between which last and the 



