182 



ECHINODERMATA— PELMATOZOA 



PHYLUM IV 



shaped bands, and the columnars are also horseshoe-shaped, not having as yet 

 formed complete rings. 



After a free-swimming existence of a few hours the embryo attaches itself 

 by means of the so-called adhesive pit, a slight depression on the antero- ventral 

 face, the cilia disappear, and profound changes take place which result in a 

 rearrangement of the internal organs. 



The five orals now form a pyramid over the superior (ventral) portion of 

 the animal, while the five basals form a similar, but inverted, pyramid, in the 

 wall of the proximal (dorsal) portion of the calyx ; between the apex of the 

 latter and the top of the column are the three or five infrabasals. The column 

 consists of about eleven cylindrical joints, each composed of the original central 

 annulus from which numerous longitudinal parallel calcareous rods are 

 developed, and is terminated distally, and attached, by a lobate terminal stem 

 plate. The larva is now said to have reached the " Cystid stage." 



In the five diamond-shaped spaces which occur between the divisions of 

 the orals and basals the radials appear, and, increasing rapidly in size, intrude 

 upon the orals ; at the same time a sixth plate (the anal) 

 makes its appearance in the zone of the radials, but it gradu- 

 ally moves upward with the orals into the ventral disk. A 

 row of elongate cylindrical segments, bifurcating on the 

 second, is given off from each radial, and grows very rapidly 

 by the addition of new plates at its distal end. The column 

 ceases adding new segments, and the last one to be formed, 

 just beneath the calyx, increases in size and fuses with the 

 infrabasals to form the rudiment of the centrodorsal. The 

 larva is now said to have reached the " Pentacrinoid stage." 

 Simultaneously with the development of the arms and 

 column a resorption of the orals and the anal sets in, while 

 the basals begin to undergo a curious metamorphosis by 

 which they are transformed into a lobate ten-rayed plate 

 which is wholly internal, lying just above the chambered 

 organ. Finally the button-shaped centrodorsal, which is now 

 beset with numerous cirri, detaches itself from the remainder 

 of the stalk, and the animal becomes free. 



The ontogeny of the Antedon (Fig. 283) reveals the fact 

 that the infrabasals, basals, orals and stem represent the 

 most primitive skeletal structures while the radials and 

 brachials are formed at a subsequent period. Similar 

 evidence is afii'orded by numerous fossil Crinoids, in which 

 the basals and column are very strongly developed, while 

 dorsal (alter Wyviiie the radials are mostly of inferior size, and the arms either 



Thomson). "^ ^ 



rudimentary or absent.^ 

 Habitat. — The existing Crinoids inhabit depths ranging from between tide 

 marks to 2900 fathoms, both extremes being occupied by unstalked forms 



^ [Some of the Paleozoic Flexibilica are almost identical, iu fact, with the pedunculate stages of 

 Antedon. Wachsmuth and Springer, from their observations on the orientation of the stem and its 

 canal in fossil monocyclic and dicyclic Crinoids, were led to infer the presence of infrabasals in the 

 nepioiiic (or larval) stages of many forms previously supposed to be without them. This predic- 

 tion was abundantly confirmed by Mr. Bury 's discovery of nuuute infrabasals in the larva of A ntedon. 

 See especially II. Bury, Early Stages in the Development of Antedon rosaceus. Philos. Trans., 

 1889, vol. clxxix.] 



Fig. 283. 



Larva of Antedon 

 rosaceus Linck. h, Ba- 

 sals ; r, Radials ; 

 o, Orals : cd, CentrO' 



