FREE-SWIMMING LARVA OF ANTEDON. 575 



fig. 1 : — b, b, the circlet of basals supported on the top of the 

 Stem ; r 1 , the circlet of first radials, now interposed hetween the 

 hasals and the orals, and alternating with both ; between two of 

 these is interposed the single anal plate, a ; whilst they support 

 the second and the third radials (r 2 , ?- 3 ), from the latter of which 

 the bifurcating Arms spring ; finally, between the second radials 

 we see the five orals, lifted from the basals on which they originally 

 rested, by the interposition of the first radials. In the more 

 advanced stage shown in Fig. 299, c, and on a larger scale in 

 Plate xxi., figs. 2, 3, we find the highest joint of the Stem begin- 

 ning to enlarge, to form the centro-dorsal plate (Fig. 2, cd), from 

 which are beginning to spring the dorsal cirrhi (cir), that serve 

 to anchor the animal when it drops from the stem ; this supports 

 the basals (6), on which rest the first radials (r 1 ) ; whilst the anal 

 plate (a) is now lifted nearly to the level of the second radiate 

 (r 2 ), by the development of the anal funnel or vent (v) to which it 

 is attached. The oral plates are not at first apparent, as they no 

 longer occupy their first position ; but on being carefully looked-for, 

 they are found still to form a circlet around the mouth (Fig. 3, o, o), 

 not having undergone any increase in size, whilst the Visceral Disk 

 and the Calyx in which it is lodged have greatly extended. These 

 oral plates finally disappear by absorption ; while the basals are at 

 first concealed by the great enlargement of the Centro-dorsal (which 

 finally extends so far as to conceal the first radials also), and at 

 last undergo metamorphosis into a beautiful ' rosette ' which lies 

 between the cavity of the centro-dorsal, and that of the calyx. — 

 In common with other members of its Class, the Antedon is repre- 

 sented in its earliest phase of development by a free-swimming 

 Larval Zooid or Psendemh-yo, which was first observed by Busch, 

 but has recently been most carefully studied by Prof. Wyville 

 Thomson. This Zooid has an elongated egg-like form, and is 

 furnished with transverse bands of Cilia, and with a Mouth and 

 Anus of its own. After a time, however, rudiments of the Cal- 

 careous plates forming the Stem and Calyx begin to show them- 

 selves in its interior ; a disk is then formed at the posterior 

 extremity, by which it attaches itself to a Sea-Weed (very com- 

 monly Laminaria), Zoophyte, or Polyzoary ; the Calyx, containing 

 the true Stomach, with its central mouth surrounded by Tentacles, 

 is gradually evolved ; and the sarcodic substance of the Pseud - 

 embryo, by which this Calyx and the rudimentary Stem were 

 originally invested, gradually shrinks, until the young Pentacrinoid 

 presents itself in its characteristic form and proportions. * 



* See Prof. Wyville Thomson's Memoir 'On the Development of Ante- 

 don rosaceus' in the "Philos. Transact." for 1865, p. 513. — The Penta- 

 crinoid Larvse of Antedon have been found abundantly at Millport, on the 

 Clyde, and in Lamlash Bay, Arran : in Kirkwall Bay, Orkney; in Lotigh 

 Strangford near Belfast, and in the Bay of Cork; and at Ilfracombe, 

 and in Salcombe Bay, Devon. 



