DEVELOPMENT OF ANTEDON (COMATULA, LAMK.) EOSACEUS. 



737 



advanced, as well as in the adult Antedon, sufficiently accounts for the ingestion of ali- 

 mentary particles, without attributing to the arms any prehensile action. Neither the 

 oral nor the brachial tentacles are ciliated ' ; but I ha^e frequently noticed a rapid move- 

 ment of particles along the radial furrows and their brachial extensions, which intervene 

 between the parallel rows of leaflets and tentacles developed on either side of the 

 tentacular canals, such as indicates the action of cilia along the floor of those furrows. 

 And as this movement alwaj-s takes -place from the extremities of the arms towards the 

 mouth, I feel little doubt that it serves the purpose of bringing within reach of the 

 ingestive current maintained by the gastric and intestinal cilia such alimentary corpus- 

 cles, as settle down upon the expanded arms and pinnules. 



2. Development of the Component Pieces of the Skeleton, from the commencement 



of the free or unattached stage. 



74. Having thus traced the history of the Pentacrinoid Larva to the stage at which 

 it assumes its adult condition as a free Antedon, I shall describe in some detail the 

 several pieces of which its skeleton is then composed ; and shall then trace the further 

 development of each to its final completion. 



75. Beginning from the base, we have first to speak of the Centro-dorsal plate, or 

 rather basin ; which, as was rightly stated by Professor J. Muller, is really the highest 

 joint of the Pentacrinoid stem; whilst, as pointed out by Professor Wyville Thomson ^ 

 "it represents a coalesced series of the nodal stem-joints in the stalked Crinoids." 

 Of this sagacious determination I shall hereafter (Part II.) adduce a most curious and 

 unexpected confirmation. The centro-dorsal plate (Plate XLI. fig 2, c) at the stage 

 now treated of, has the form of a basin with its lip mverted instead of everted ; its 

 diameter is about -03 inch, and its height about -012 inch. Its basal surface is some- 

 what depressed in the centre ; and here we may for a time distinguish a minute five- 

 rayed perforation (fig. 6, c), which previously formed the communication between the 

 cavity of the basin and the central canal that is still left in the upper segments (at 

 least) of the stem. This perforation, however, is very soon closed up by an extension of 

 the calcareous network, so that no trace of it remains visible either exteraally or inter- 

 nally. Around the stellate aperture is seen a circular series of five sockets for the arti- 

 culation of the dorsal cirrhi, each of them having a pore in its centre, which is usually 

 at the summit of a minute elevation. This pore gives passage to a sarcodic thread which 

 proceeds from the sarcodic axis contained within the cavity of the basin, and runs along 

 the central canal of the cirrhus to its termination. A second series of sockets, alternating 

 in position with the first, is seen nearer the upper margin of the basin. This margin, 



' Mr. J. V. TnoMrsoif speats of the brachial tentacula as '• furnished with capitate cilia, nltcniatoly placed 

 along their sides : " but it is clear that ho referred to tlio tubular processes of the tentacles -n-hich liuvo been 

 described by Trofcssor Wyvillk Thomson (p. 520), and that, vrriting in 1827, he did not employ the term cilht 

 iu the technical sense to which it is now limited, but merely meant distinct hair-lilcc filaments. 



' Philosophical Transactions, 1805, p. 530. 



5n2 



