64 ME. P. H. CAEPENTER OX THE GENUS ACTINOMETRA. 



one third of the total diameter of the plate. The floor of this cavity is marked hy minute 

 punctations (u), which are the internal orifices of canals proceeding from it towards 

 the dorsal surface of the plate. They originally opened externally on the summits of 

 the small tubercles occupying the centres of the sockets for the articulation of the 

 first developed cirrhi in the young animal ; hut their openings have gradually become 

 obliterated by the deposit of new material upon the central portion of the external 

 surface of the plate, as described by Dr. Carpenter ' in Antedon rosacea. This is 

 accompanied by the continual removal of old material from the internal surface, so that 

 the minute openings (PL IV. fig. 15, u) seen on the central part of the floor of the 

 internal cavity of the centrodorsal piece are the original external openings of the first 

 developed canals, which have subsequently become closed externally by the new material 

 deposited upon the central part of the dorsal surface. The internal openings of the 

 canals proceeding to the last developed cirrhi are much larger, and placed more towards 

 the periphery of the floor of the cavity. Similarly in Actlnomelra polymorpha, the 

 internal openings of the canals proceeding to the existing marginal cirrhi on the plate- 

 like centrodorsal piece are placed under its projecting rim, so as not to be visible from 

 above. There are usually one or two large openings under the central margins of each 

 of the radial areas (PI. VI. figs. 3, 8, 10, 15, 17, 21, r.ar), and the canals which proceed 

 outwards from these internal openings break up into five branches, one of which reaches 

 the summit of each of the small tubercles occupying the centres of the five cirrkus- 

 sockets, which are placed along the outer or peripheral margin of each of the corre- 

 sponding radial areas (PL VI. figs. 1, 2, 7, 14, 16, 20, 77). These canals enclose the axial 

 cords of the cirrhi (PL VIII. figs. 1, 3, 4, 5, G, 8, n.c), which proceed from the fibrillar 

 envelope of the quinquelocular organ contained in the cavity of the centrodorsal plate 

 (PL VIII. figs. 1, 2, 3, 7, -V), and surround the cirrhus vessels arising from its chambers 

 (figs. 2, 3, 7, ch), from each of which there arises a single trunk ", dividing, sooner or 

 later, into branches for the individual cirrhi. In the specimen of var. 1 represented in 

 PL VI. fig. 15, the division is not completed within the cavity of the centrodorsal plate, 

 as two or, sometimes, even only one aperture can be seen under the inner margin of 

 each of the radial areas, so that the primary trunk enters the substance of the plate, and 

 there divides into the five branches for the cirrhi placed on the outer margin of each 

 radial area. 



The rim of the cavity of the centrodorsal plate of Actinometra polymorpha, var. 1 

 (PL VI. fig. 15, cd.c), is ten-sided, or nearly circular, and is not marked by shallow 

 radial depressions, like those described above in Ant. rosacea (PL IV. fig. 15, q). The 

 radial areas rise very slightly from their peripheral to their central margins, and are 

 marked by various indistinct ridges and furrows. Their sides rise towards the five inter- 

 radial elevations, which, though not very much raised above the general surface of the 

 plate, are nevertheless very distinct ; for they are wide and marked by shallow grooves 



1 Phil. Trans, lot. tit. pp. 742, 743. 



2 This is in precise accordance with the origin of the vessels proceeding to the cirrhi which arc borne on tho stem 

 of Pen.tacrinus. At every nodal segment the five chambers which are placed radially around the central axis of the 

 stem enlarge slightly, and each gives off a single vessel to one of the five cirrhi. 



