ME. P. H. CARPENTER ON THE &ENUS ACTINOMETRA. 63 



var. 1, since it is not reduced in size by the truncation of its edges, as is the case iu 

 these forms. 



(§ 42) The meaning of these processes becomes apparent when we examine the 

 ventral or superior surface of the centrodorsal plate which bears the superjacent radial 

 pentagon. Its condition in Act. poJijmorpha is rather complicated, and will be better 

 understood after an examination of the simpler forms of this surface presented by other 

 species of Comatula. 



In the large variety of Ant. celtica, the cavity of the centrodorsal piece (PI. IV. 

 fig. 2, ccl.c) is very deep, and its opening on the flattened ventral surface has a circular 

 or somewhat pentagonal form. Between the angles of this internal pentagon and those 

 of the outer more distinctly pentagonal margin of the piece run five slight ridges or 

 elevations [l.e). In the intervening radial areas {r.ar), between these ridges the surface 

 is somewhat depressed, so as to receive the convexities of the dorsal surfaces of the first 

 radials that rest upon it, while the five ridges correspond with five slight furrows 

 marking the lines of junction of these surfaces on the dorsal aspect of the radial pen- 

 tagon (PL IV. fig. 3), and are therefore interradial in position. 



In Ant. celtica these ridges are tolerably uniform in width throughout their whole 

 course ; but in Ant. rosacea (PI. IV. fig. 15) they are considerably wider at their internal 

 or central ends than they are towards the external pentagonal margin of the piece, so 

 that they have an elongated triangular form. From the base of each triangle a shallow 

 dej)ression extends a little way towards its apex, cut out along the median line of the 

 ridge ; but it soon becomes obliterated by the gradual aj)proximation of its two sides, 

 which meet and form a simple ridge, like that of Ant. celtica (PI. IV. fig. 2), extending 

 outwards to the margin of the plate. 



The central ends of the radial areas into which the ventral surface of the plate is 

 divided are marked in Ant. rosacea by five shallow depressions (PI. IV. fig. 15, q), 

 placed close to the margin of the internal cavity, which bends somewhat inwards at 

 these points. These depressions correspond in position with five large radial openings 

 on the dorsal surface of the pentagonal base of the calyx (PI. IV. fig. 16, Q), and receive 

 the blind ends of five diverticula of the body-cavity, which are enclosed between the five 

 radial spout-like processes of the rosette (PI. IV. figs. 13, 16, ^j) aud the internal faces 

 of the first radials. They are, however, occasionally absent in Ant. rosacea, while, on the 

 other hand, traces of them may occur in Ant. celtica. In fact, the diff'erences which 

 I have described above in the appearance of the ventral surface of the centrodorsal piece 

 of these two species must not be regarded as representing more than two extreme 

 variations of one and the same type. 



(§ 43) We shall now be able to understand the meaning of the short processes (^) 

 above mentioned, which are seen projecting from the angles of tlie centrodorsal plate of 

 Act. polymorpha, when viewed from below. It will be best to begin with the examina- 

 tion of the ventral surface of the centrodorsal piece of variety 1 (Pi. VI. fig. 15), in 

 which they are more distinctly marked than in the type. This surface rises slightly 

 from the circumference towards the centre, which is occupied by the opening of a 

 shallow cavity, the centrodorsal coelom {ccl.c), the diameter of which is rather less than 



