42 ME. P. H. t'AKPENTEK ON THE GENUS ACTINOMETRA. 



disposed along the edges of the primary groove-trunks proceeding from it, and there 

 are generally some upon the sides of the anal tube. They are particularly well de- 

 veloped in the dark variety from Ubay, in which all the arms are tentaculiferous. 



(§ 28) The "oral pinnules" of Act. polymorpha, those, namely, which arch over the disk 

 so as to protect it, are borne by the second distichals and second palmars when these are 

 present, but in any case upon the second brachials, those of the distichals and palmars 

 being the longest. They are all very long and slender, consisting of some 30 or 40 

 segments ; and their terminal portions exhibit the peculiar characteristic comb made up 

 of processes which rise from the outer margin of the ventral surface of each calcareous 

 segment (PI. III. fig. 2), just as in Act. Solaris and Act. pectmata (PI. III. fig. 1). The 

 number of segments on which these processes may be developed varies from 10-12 on 

 a distichal pinnule, to 6-8 on a brachial pinnule ; but in cases in which no second 

 distichals or palmars are developed, so that the pinnule on the second brachial is the 

 first of the series, it is much longer than usual, and more of its terminal segments bear 

 the comb-like processes. 



The oral pinnules of the dark Ubay variety of Act. polymorpha differ considerably 

 from those of the type and of other Actmometrce ; not only are they much stouter, but 

 their terminal comb is differently constituted (PL III. fig. 3). As is usually the case, the 

 lower processes gradually develope themselves from the outer margin of the ventral 

 surface of each calcareous segment ; but towards the end of the pinnule they gradually 

 come to rise less and less from the outer margin, and more and more from the median 

 portion of the ventral surface of each segment, until finally, on the last two or three 

 segments, they are developed from the inner margin. Consequently the comb, when 

 viewed from above, is seen not to lie altogether on the outer side of the pinnule, as is 

 usually the case, but to start from the outer side, cross its ventral surface, and finally 

 come to lie on the inner side of each pinnule, i. e. on the one nearest the arm. 



Both in the type of Act. polymorpha and in all the four varieties, the pinnules 

 diminish in length from that of the second distichal (when present) to those borne by 

 the fourth and fifth brachials; that of the sixth brachial is longer, and usually contains 



well-developed genital gland, so that it is slightly swollen. Prom this point onwards 

 the pinnules increase in length till about the thirtieth brachial, after which their length 

 and character vary according as the arm is tentaculiferous or non-tentaculiferous. 



(§ 29) The dorsal aspect of Act. polymorpha differs from that of most Antcdons, and 

 especially from that of Ant. rosacea, in the fact that the plane of the second and third 

 radials, like that of the first, is parallel to the vertical axis of the calyx, and not inclined 

 to it, as in Antedon ; so that the dorsal surfaces of the whole of the pieces of the calyx 

 lie in one horizontal plane. The centrodorsal piece is circular (PI. II. figs. 9, 10, c d), or 

 pentagonal (fig. 11), and conceals a large portion of the pentagon formed by the first 

 radials, less in young specimens with but a few arms (fig. 9) than in large and full-grown 

 specimens with many arms (PI. II. figs. 10, ] I & PI. VI. fig. 2). It is usually a flattened 

 plate with a slight concavity in the centre of its outer surface; and around its margins 



