ME. P. H. CAEPENTER OlS THE GENUS ACTINOMETEA. 101 



as ^vill be seen by comparing PI. IV. fig. 9 with PI. V. fig. 13, both of wbich rejjresent 

 the internal aspect of two united first radials. In Actinometra (PI. V. fig. 13) there is 

 an abundant calcareous network {c.n) in connexion with the internal faces of the radials, 

 which are marked by well-developed radial and interradial farrows. In Ant. Eschrichtii, 

 however (PL IV. fig. 9), the processes forming this network are but little developed ; 

 there is no axial interradial furrow, and even the radial one is indistinct, except near 

 the dorsal surface, where it passes between the raised edges of the two apertures 

 {x', [I, y', z) of the central canal which unite with the inflected edges of a radial spout- 

 like process (p) of the rosette. 



The interradial process of the rosette (o') is short and broad, but without the spout- 

 like character which it has in Actiuomefra — being simply directed, as in the normal 

 condition of Ant. rosacea (PI. IV. figs. 3, 7, 16), to the line of suture between the two 

 contiguous radials, to which it is attached between the two adjacent apertures (fig. 9, y, y') 

 of their central canals. 



(§ 69) The remarkable variation in the extent to which the rays of the basal star may 

 be developed, as described above in Actinometra and in Antedon Esclirichtii, is due to the 

 fact that tliey are not calcifications in a nucleated protoplasmic network like the ordinary 

 elements of the skeleton. They are the result of a calcareous deposition, of a more 

 or less regular character, around the connective-tissue fi.bres which effect the synostosis 

 with the centrodorsal piece of every pair of contiguous radials along the line of 

 contact of the latter ; so that their position is, as we have already seen, interradial. 



In PI. III. fig. 5 is seen the lower end of a vertical section cut transversely to the plane 

 of synostosis of two decalcified first radials (ri) oiAct. polymorpha, close to their peripheral 

 margin where they are not concealed by the centrodorsal piece, so that the fibres of the 

 elastic ligaments connecting them with tlie second radials are cut somewhat obliquely (/J. 



The threads of the protoplasmic network of which the organic basis of the radials is 

 composed pass somewhat rapidly at their sm*faces into the connective-tissue fibrils (i) 

 which run horizontally between them and effect the synostosis. These fibrils being very 

 closely set, the suiierficial portions of the calcareous reticulation forming the skeleton 

 of the radials, which are ossified around their ends, are very much more dense than the 

 central portion produced by calcification of the protoplasmic network. 



PL VIII. fig. 4 represents a section, from the same series as the previous one, across 

 tlie line of union of the same two radials {A, B), rather nearer to the centre of the calyx, 

 so that their dorsal surface appears no longer free, but partially covered by the centro- 

 dorsal piece {cd). The lower portion of this section, more highly magnified, is seen in 

 PL III. fig. 6. The synostosis of the radial areas of the centrodorsal piece with the 

 dorsal surfaces of the first radials is effected by simple and not specially numerous con- 

 nective-tissue fibres {I) ; these pass directly from the protoplasmic basis of the one piece 

 into that of the other, in a direction vertical to the plane of the opposed surfaces, just 

 as in an ordinary synostosis. But in the direction of the interradii the course of the fibres 

 is different, and they have a deeper origin in the substance of the centrodorsal piece 

 than those which occupy the radial areas (PL III. fig. 6). 



There are three principal masses of these longer interradiaUy placed fibres : — two smaller 



14i* 



