ECIIINODERMA TA. 



477 



the stem next the calyx, and a new element the radials makes its 

 appearance as a ring of five small plates, placed in the space between 

 the basals and orals, and in the intervals 

 alternating with them (fig. 270, 4). The 

 roof of the oral vestibule (vide fig. 253 and 

 p. 460) has in the meantime become rup- 

 tured; and the external opening of the 

 mouth thus becomes established. Sur- 

 rounding the mouth are five petal-like 

 lobes, each of them supported by an oral 

 plate (fig. 268 C). In the intervals be- 

 tween them five branched and highly con- 

 tractile tentacles, which were previously 

 enclosed within the vestibule, now sprout 

 out : they mark the position of the future 

 radial canals, and are outgrowths of the 

 water- vascular ring. At the base of each 

 of them a pair of additional tentacles is 

 soon formed. Each primary tentacle cor- 

 responds to one of the radials. These 

 latter are therefore, as their name implies, 

 radial in position ; while the basals and 

 orals are iuterradial. In addition to the 

 contractile radial tentacles ten non-con- 

 tractile tentacles, also diverticula of the 

 water-vascular ring, are soon formed, two 

 for each interradius. 



In the course of the further develop- 

 ment the equatorial space between the 

 orals and the basals enlarges, and gives 

 rise to a wide oral disc, the sides of which 

 are formed by the radials resting on the 

 basals ; while in the centre of it are placed 

 the five orals, each with its special lobe. 



The anus, which is formed on the 

 ventral side in the position of the blasto- 

 pore (p. 460), becomes surrounded by an 

 anal plate, which is interradial in posi- 

 tion, and lies on the surface of the oral 

 disc between the orals and radials. On 

 the oral plate in the next iuterradius is 



placed the opening of a single funnel leading into the body cavity, 

 which Ludwig regards as equivalent to the opening of the madreporic 

 canal (vide p. 460) \ 



1 I have made no attempt to discuss the homologies of the plates of the larval 

 Echinoderniata because the criteria for such a discussion are still in dispute. The 

 suggestive memoirs of P. H. Carpenter (No. 548) on this subject may be consulted 

 by the reader. Carpenter attempts to found his homologies on the relation of the 



FIG. 270. YOUNG PENTACEI- 



NOID LABVA OF ANTEDON. (From 



Carpenter; after Wyville Thom- 

 son.) 



1. terminal plate of stem; 

 cd. centro-dorsal plate; 3. ba- 

 sals; 4. radials; or. orals. 



