1879.] the Comatulge of the il Challenger'" Expedition. 393 



taking up more than half the whole dorsal face of the radial, while in 

 the younger specimen it occupies much less than half. 



The effect of this change in the component parts of the radial 

 pentagon is to give its central synosteal surface a considerable slope 

 inwards and downwards, so that the whole, when viewed from above, 

 has the form of a wide and shallow funnel. The rim of the funnel 

 (outer dorsal surfaces of the radials) is thick in the young specimen, 

 but does not increase with the growth of the interior (inner dorsal 

 surfaces). Consequently, the centrodorsal which forms, as it were, a 

 plug fitting into the funnel, slips farther and farther down into it, until 

 its dorsal surface becomes level with that of the radial pentagon, or 

 even comes to be actually below it. At the same time it loses its few 

 marginal cirrhi, and their sockets become obliterated, so that the 

 whole dorsal surface of the calyx is one uniform plane. Act. Jukesii 

 remains permanently in this condition ; but there are other species, as 

 we have seen, and notably Act. stellata, in which the centrodorsal loses 

 its pentagonal shape, owing to the appearance of more or less deep 

 clefts between its outer edge and the inner edges of the radials. 



In Act. pectinata the ventral face of the centrodorsal is divided by 

 ridges into five radial areas, corresponding with the five synosteal 

 surfaces of the first radials that rest upon it. These radial areas are 

 occupied by median depressions, which increase somewhat in depth 

 from their peripheral to their central ends. But the synosteal sur- 

 faces of the radials do not exhibit corresponding ridges, for they are 

 marked by similar median depressions, which are also deepest at their 

 central ends. When, therefore, the synosteal surface of the radial 

 pentagon and the ventral surface of the centrodorsal are in their 

 normal state of apposition, they are separated from one another along 

 the median lines of the five radials by five cavities or " radial spaces." 

 Those are largest at their blind central ends, and extend in a peri- 

 pheral direction to open externally by five minute openings, situated 

 round the margin of the small centrodorsal piece, beneath the radial 

 pentagon which rests upon it, and extends considerably beyond it. It 

 seems to me that we have here an explanation of the large openings 

 between the radials and centrodorsal of Act. stellata and Phano- 

 genia, &c. In Act. pectinata these radial spaces end blindly around 

 the central cavity of the radial pentagon, being shut off from it by the 

 thickened inner margin of its synosteal surface. Whether they are 

 also blind in Act. stellata, in which they are so very large, or whether 

 they are in communication with the radial diverticula of the ccelom, 

 which are inclosed within the spouts of the rosette, is a point which 

 can only be settled by making a series of sections through the decal- 

 cified calyx. 



I have elsewhere (Actinometra, cap. iv, § 61) drawn attention 

 to the homology of these openings between the radial pentagon and 

 vol. xxvm. 2 G 



