280 



PHYLUM ECHINODERMATA. 



anus of Holopus is not known). In the living genera Hyocrinus 

 and Thaumatocrinus, the orals are very similar to those of the 

 Larviformia, but between them and the edge of the calyx 

 there are a few rows of interambulacral plates (Fig. 209). In 

 Rhizocrinus also the orals are present, but much smaller than in 

 those first mentioned, the part of the calyx-cover occupied by 

 interambulacral plates being still more extensive. 



In the Larviformia the oral pyramid is supposed to have been 

 closed, the oral plates being actually connected with one 

 another along the lines of contact, except at the base of the 

 arms, where a small opening exists through which the ambu- 

 lacral grooves pass out on to the arms. In such cases the mouth 



is said to be subtegminal. This condition 

 is not found in any living form : in 

 Holopus, Hyocrinus, etc., the oral plates 

 are not continuous and the mouth opens 

 between them. But a very similar state 

 of affairs is found for a short period in the 

 development of Antedon. In the larva of 

 this animal there is a stage in which the 

 mouth surrounded by its fifteen tentacles 

 opens into a closed sac, the oral vestibule 

 in the walls of which are contained the 

 large oral plates (p. 157). In the majority of forms, however, 

 both living and extinct, the orals are either comparatively in- 

 conspicuous or absent in the adult. 



In the Articulata (except Rhizocrinus) orals are absent. In the Fistu- 

 lata they are distinct in some forms and not in others. When they are 

 distinct they are in the centre of the calyx-cover and the posterior is the 

 largest and is placed partly between the others. In. some forms with 

 subtegminal mouth (some Cyathocrinidae) the mouth is covered by 5 

 proximal ambulacral plates which simulate orals, the true orals (deltoids) 

 being outside these. In the Flexibilia they are present in Taxocrinus 

 and surround the open mouth (Fig. 195). In the Camerata they can 

 generally be distinguished at the apex of the vault into which the calyx- 

 cover is produced, but their identification is sometimes uncertain. 



In the forms with small orals the greater part of the calyx - 

 cover is occupied by compactly or loosely arranged interambu- 

 lacral plates, which are continuous with the interradial plates of 

 the calyx, if such are present, and by the ambulacral grooves 

 iii their passage outwards to the arms. 



FIG. 195. Calyx cover of 

 Taxocrinus intermedius 

 (after W. and Sp., from 

 Delage and He'rouard). 



