ME. P. II. CAEPENTEE ON THE GEjSTJS ACTIXOMETEA. Go 



(PI. VI. fig. 15, h.g), which occupy the greater part of their width, so that the simple 

 ridge, as seen in Ant. celtica (PI. IV. fig. 2, i.e), is here represented by the two sides of 

 the groove which is cut out along its median line. In Ant. rosacea (PL IV. fig. 15), as 

 we have already seen, these sides meet at a very short distance from the central eud of 

 the groove so as to obliterate it. In this form, however, they approach one another very 

 gradually, and only just meet within the margin of the plate; but the ridge formed by 

 their fusion does not end here as in A)it. rosacea, for it is continued a short distance 

 beyond the general surface of the plate, so as to appear as a short process {t) extending 

 outwards from the angle between two sides of its external pentagonal margin. Conse- 

 quently these five short processes appear on the dorsal aspect of the plate, prolonging its 

 angles outwards, as we have seen in sect. 41 (PI. VI. fig. 14, t). 



(§ 41) The grooves {h.fj) which are thus cut out along the median line of the inter- 

 radial elevations on the ventral surface of the centrodorsal plate of Actinometra are of 

 no little importance ; for there lie in them, as will be seen further on, the five rays of the 

 basal star (PL VI. fig. 13, 8), which is in close connexion with the dorsal surface of the 

 radial pentagon ; they may therefore be called the " basal grooves " {h.g). 



As a general rule, these interradial elevations and the basal grooves are, like the 

 rays of the basal star, entirely devoid of pigment, which is, however, very abundant in 

 the organic basis of the calcareous reticulation composing the rest of the ventral surface 

 of the plate ; so that when this is first exposed by the removal of the centrodorsal from 

 the dorsal surface of the radial pentagon which rests upon it, five white rays are visible 

 on a dark background. Unless the plate is immediately removed from the alkaline 

 solution used to efiect its separation, this distinction in colour between the radial and 

 the interradial portions of its ventral surface rapidly disappears, owing to the destruc- 

 tion of the pigments contained in the former. 



The development of these basal grooves is not only different in the type and in all the 

 varieties of Act. polymorpha, but it differs in different individuals of the type, and even 

 to a certain extent in the same individual. 



In the specimen of the type represented in PL VI. figs. 7, 8, which, like variety 1, 

 had 25 cirrhi, two only of the basal grooves are seen ; for the other three are occupied by 

 the rays of the basal star (fig. 8, S), which have become detached from the rest of the 

 star and from the first radials with which they were connected. But even these two 

 grooves do not resemble one another ; one extends almost to the margin of the plate, 

 beyond which the interradial ridge formed by the union of its sides is continued as a 

 short process {t), just as in var. 1 (fig. 15). The other open groove, however, terminates 

 very soon, as its sides, widely separated at its central eud, bend sharply towards one 

 another, and meet some distance witliin the margin of the plate, to which the ridge 

 formed by their union does not extend, for it terminates abruptly in a blind and 

 rounded extremity. 



In variety 1 (PL VI. fig. 15) the basal grooves {b.g) are narrow, and after increasing 

 a little at first, diminish gradually in width from their central to their periplieral ends ; 

 but in the specimen of the type (fig. 8) they are much wider in proportion to theu* 

 length, and the width increases slightly from then- base to about the middle of their 



