62 2 ECHINODERMATA 



CHAP. 



is the fixed stage retained, and both Ophiuroidea and Echinoidea 

 pass through an Asteroid stage in development. The only serious 

 competitors for the position are the Holothuroidea, which many 

 have imagined to have been directly derived from Cystoidea (in 

 the old sense ; better Thecoidea). This view, though adopted 

 by Semon, 1 Haeckel, 2 and Bather, 3 is open to many objections. 

 The type of Holothuroid development referred to in these dis- 

 cussions is that of the extremely aberrant Synapta digitata, in 

 which the radial canals are vestigial structures which disappear 

 in the adult. In this species, where the feelers are multiplied, 

 some originate in the larva directly from the water-vascular 

 ring, and thus alternate with the canals. From this circum- 

 stance Semon drew the conclusion that the radial canals of 

 Holothuroidea are not homologous with those of other Echino- 

 derms, but this conclusion is contradicted by the development 

 of more normal species, in which all the feelers spring from 

 the radial canals. The meridional course of these canals, the 

 closure of the ambulacral grooves, coupled with the retention of 

 a nervous ectoderm, are all features found in Echinoidea. So is 

 also a reduction in the number of the genital organs, on which 

 Bell 4 laid such stress that he separated Holothuroidea from all 

 other Echinoderms. But if in Spatangoidea a reduction to four 

 and even three can take place (Fig. 246, p. 552), why should a 

 reduction to two or one excite surprise ? The primitive outer 

 appearance of the Auricularia is counterbalanced by the develop- 

 ment of the coelom, which is much modified, so that the primitive 

 bilateral arrangement is obscured. If, then, Asteroidea are the 

 most primitive Eleutherozoa, we may imagine that primitive 

 Echinoidea were derived from Asteroidea through adaptation 

 to life in crevices, where an upward bending of the radii 

 was of advantage, in order to enable the animal to attach its 

 podia above as well as below itself; and that Holothuroidea 

 arose from primitive Echinoidea in which the plates of the 

 corona were still movable, through a further adaptation to 

 narrow crevices, where worm-like wriggling would be the most 

 successful method of adapting themselves to their environment. 



1 " Die Entwicklung der Synapta digitata," Jen. Zcitschr. xxii. 1888, p. 175. 



2 " Die Cambrische Stammgruppe der Echinodermen," Jen. Zcitschr. xxx. 1895. 



3 Lankester's Treatise on Zoology, " Echinodermata," pt. iii. 1900, p. 33. 



4 Brit. Mus. Cat. "British Echinodermata," 1892, p. 14. 



