viii EGHINODERMATA—PHYLOGEXY 545 



to the Inadunata, especially to the so-oalled Larviformia (c/. pp. 303, 328, etc.), 

 is so striking as to be at once recognisable. 



The calyx, with the arms, sooner or later breaks away from the stalk, and 



can either move !>)' using the arms as paddles or catch on to objects by means of its 

 cirri. When it breaks loose from the stalk, some of the uppermost whorl joints on 

 which cirri have formed remain connected with it ; these fuse with one another, and 

 with the centrodorsal. The basals, again, fuse to form a rosette, which is soon over- 

 grown on all sides by the large apical centrodorsal plate. 



XXII. Phylogeny. 



No other phylum of the animal kingdom is so sharply marked off from all others 

 as the Echiuoderms. Their organisation is in all points strange ; even the radiate 

 structure is strange, in so far as it is, unlike that of many Ocelentcrata, only a mask 

 which hides a complicated and hitherto inexplicable asymmetry. We are not in a 

 position to compare an adult Echinoderm with the adult representative of any other 

 phylum from a phylogenetic standpoint. 



The difficulties which meet us in attempting to reconstruct the phylogenesis of 

 the Echinodermata are still fui'ther increased by the fact that the typical charac- 

 teristic Echinoderm larva cannot at any stage of its development be compared with 

 the adult or larval form of any other animal. An exception to this statement may, 

 however, perhaps, be made in favour of the Enteropncusta, which will be described in 

 the next chapter. 



If, taking the gastrsea theory as a foundation, we assume for the Metazoa a 

 common bilaminar racial form, it seems, in view of the above-mentioned difficulties, 

 that the racial form of the Echinodermata must have branched off extraordinarily 

 early, perhaps at a stage corresponding phylogenetieally with the gastrula. By such 

 an assumption, the Echinoderms and their larvse would be removed from the sphere 

 of comparative anatomy and comparative embryology, except in so far as such com- 

 parative enquiry were limited to the Echinoderm phylum itself. 



It appears to us, however, that attempts to approximate the Echinodermata to 

 Metazoa standing higher than the Ccelenterata should not be abandoned. Recent 

 anatomical and ontogenetic researches have brought to light facts which open up new 

 prospects. We may mention the demonstration of a neural plate and of a larval 

 nervous system, the attempts to demonstrate that there are two pairs of enterocoel 

 vesicles, the proof that the first rudiments of the gonads proceed from the endothe- 

 lium of the ca4om, the suggestion that the stone canal or the hydropore should be 

 regarded as a nephridial canal, etc. 



All this, of course, does not justify us in clo.sely comparing the Echinoderm larva 

 with other definite forms, adult or larval, belonging to Metazoan classes higher than 

 the Ccelenterata, except perhaps the Enteropncusta. But these discoveries and new- 

 views tend to make the Echinoderm body appear somewhat less strange, since we 

 find in its organisation important points in which it is fundamentally in agi'eenient 

 with the so-oalled Ti-iploblastica. 



It cannot be doubted, and has never been doubted, that the Echinodermata form 

 a distinct, naturally marked out phylum of the animal kingdom, or, in the language 

 of Phylogeny, that all Echinoderms have had a common racial form. 



Within the phyhun of the Echinodermata, further, the classes are again quite dis- 

 tinct and naturally marked off from one another. Among known Echinoderms, 

 there are no intermediate forms between the Pelmatozoa, the Holothurioidea, the 

 Echinoidca, the Asteroiden, and th-e Ophiuroidea. Every known Echinoderm can at 

 once be recognised as either an Echinoid, an Asteroid, a, Hohlhurid, etc. The Cystidea 

 VOL. II 2 N 



