MR. P. H. CARPENTER ON THE GENUS ACTINOMETRA. 103 



right of the figure are seen portions of three first radials (t\), the central parts of which 

 are lighter than the more peripheral parts, as the section here passes through the slightly 

 developed unpigmented fibrous tissue (I) connecting the radials with the centrodorsal 

 piece (cd), the peripheral portion of which consists of the same pigmented protoplasmic 

 network as the substance of the radials. 



Two of the synostoses between the latter are seen at the top of the figure ; but they 

 do not quite reach to the centre, where their place is occupied by the central ends of two 

 of the rays of the basal star (#1), the remaining three rays of which are visible in the 

 lower part of the figure for the greater portion of their length. Two of them are also seen 

 in the left or more dorsal portion of the next section (fig. 2), which also shows three 

 synostoses (L) between the radials in the right or more ventral part of the figure. 



The sections of these rays of the basal star appear very dark, not from the presence 

 of pigment, which is entirely wanting in their fibrous basis, but because of the abund- 

 ance and very close approximation of the fibres of this basis ; these are the diverging 

 and vertical fibres seen in PI. VIII. figs. 3, 4, and more highly magnified in PI. III. 

 fig. 6 (S u S 3 ). Just in the same way, in PI. VIII. fig. 3, the section appears much darker 

 on the right-hand side, where it cuts the closely approximated connective-tissue fibres 

 effecting the synostosis between two first radials transversely, than on the left-hand side, 

 where it passes through the more open protoplasmic network of the individual segment 

 of a single radial, I). 



(§ 70) This great development of fibrous tissue along the interradial portions of the 

 centrodorsal piece and of the pentagonal base of the calyx accounts for the fact, often 

 mentioned already, that there is no pigment in the substance of the rays of the basal 

 star (which is a more or less complete calcification of the central portions of these inter- 

 radial fibrous masses), nor in the walls of the basal grooves on the centrodorsal piece, nor 

 in those of the dorsal interradial furrows on the inferior surface of the pentagonal base, 

 which are calcifications of the smaller lateral masses of long fibres running directly from 

 the organic basis of the centrodorsal piece into that of the first radials (PL III. fig. 6, 6' 3 ). 

 These lateral fibres have a common point of origin in the substance of the centrodorsal 

 with the vertical and diverging fibres (#„ S 2 ), around which the calcareous tissue of the 

 basal rays is deposited. It is therefore easy to understand that the calcification may 

 in some cases be so complete that the basal rays formed around the median fibres (S 1} S 2 ) 

 may become completely united with the walls of the basal grooves formed around the 

 lower ends of the two lateral fibrous masses (*$'.,) ; as is the case in the specimen of Act. 

 polymorpha represented in PL VI. fig. 8, where two of the rays of the basal star (S) are 

 so completely united with the floor and sides of the basal grooves in which they lie that 

 the line of junction between them becomes indistinguishable. 



The fact that the rays of the basal star are calcifications in connective tissue and not 

 in the ordinary nuclear tissue which forms the organic basis of the other parts of the 

 skeleton, also affords an explanation of the great variations in the extent to which the 

 rays are developed. The general arrangement of the fibres constituting the interradial 

 portions of the synostosis between the centrodorsal piece and the radial pentagon is essen- 

 tially the same in Antedon as in Actinomctra. In Ant, rosacea and Ant. celtlca they 



