REPORT ON THE CRINOIDEA. 7 



separates itself from the stem-joint below it. and the " head " of the Pentacrinoid larva 

 becomes a free-swimming Feather-star, the rest of the larval stem being left to waste 

 away. The precise epoch of growth at which this separation occurs varies greatly. Thus, 

 for example, the young Antedon tenella retains its stem until twenty or thirty cirri have 

 appeared on the centro-dorsal, which conceals the basals, and the pinnules are developed 

 upon all the lower arm-joints ; whereas in Antedon rosacea and in other species, the stem 

 is discarded when there are only ten cirri on the centro-dorsal, the basals are still visible, 

 and the lowest portions of the arms devoid of pinnules ; while the absolute size which is 

 reached by the mature larva before dropping off its stem varies considerably. 



After the formation of the first two whorls of cirri no special regularity can be traced 

 in the manner of their development. The young ones normally appear between those 

 previously formed and the radial pentagon, so that their sockets are close to the margin 

 of the centro-dorsal (PI. I. fig. let; PL II. figs. 2a, 4a; PL IV. figs, la, 3a). But 

 as the centro-dorsal grows and new cirri appear round its margin, the older cirri which 

 are attached close to the dorsal pole drop away, and their sockets become gradually 

 obliterated by calcareous deposit. The earlier stages of this process are seen in PL I. 

 fig. 6a; PL II. figs, la, 3a, 5a; and PL III. figs. 6a 7 , 7a; and the result is that the 

 dorsal surface is usually left comparatively smooth, as seen in PL IV. figs, la, lb, 2a, 3a, 

 but in some species of Antedon the deposit of new material continues after the cirrus- 

 sockets are obliterated, and causes the dorsal pole to become rough and irregular (PL III. 

 figs. 4b, 5a ; PL XL fig. 3). On the other hand, the lower surface of the centro-dorsal in 

 most species of Actinometra is almost flat and extremely smooth (PL V. figs. 16, Id, 2b, 

 2d, 2e, 4b, 5b, 5c). This is owing to the very extensive and uniform manner in which 

 the new material is deposited, and it sometimes produces very singular results, as will 

 be explained subsequently. 



During the Pentacrinoid stage of larval existence the young Comatula is provided 

 with a stem which encloses a neuro- vascular axis just as in an ordinary Stalked Crinoid. 

 This axis contains the downward extensions of the peripheral cavities of the chambered 

 organ within the centro-dorsal and of its central axis. When the centro-dorsal separates 

 itself from the lower part of the larval stem, a minute five-rayed perforation remains at 

 its dorsal pole, which corresponds to the central canal in the stem of a Pentacrinus, and 

 gave passage to the neuro-vascular axis above mentioned. In recent Coinatulse this 

 opening is closed up very soon after the entry upon the free stage of existence, by a 

 portion of the calcareous deposit already noticed ; though traces of it are sometimes 

 visible internally upon the floor of the centro-dorsal cavity (PL II. figs. 2b, 3b). There 

 are some fossil Comatulae, however, in which it seems to have remained permanently 

 open throughout life, so far as we can judge from the material at our disposal ; while 

 in other forms again it is extended into a large stellate impression which occupies a 

 considerable space on the lower surface of the centro-dorsal, and in the fossil condition is 



