ECHINODERMA GENERAL DESCRIPTION 



o- 



s.c. 



l.hc 



M 



suggest that all Echinoderma are descended from sessile ancestors 

 (necessarily representing a stage subsequent to the Dipleurula), and 

 that the oldest among these had not acquired radial symmetry, that 

 being, it would appear, a consequence of fixation. Fixation was 

 retained more or less completely by Cystidea, Blastoidea, Crinoidea, 

 and Edrioasteroidea ; but among the other classes it is only the 

 Stelleroidea that now preserve traces of it in their ontogeny. 



The passage from the Dipleurula to the fixed stage is best 

 studied in Antedon (Bury, 1888; and Seeliger, 1893); but even 

 here changes that, in phylogeny, must have succeeded fixation 

 now precede it, and actually precede the free-swimming stage of the 

 larva. Fixation takes place by a modified portion of the preoral 

 lobe (p.l), as also in Stelleroidea. 

 The phylogenetic result of this was 

 the passage of the mouth (0) to 

 the posterior end of the Dipleurula, 

 which was now directed upwards 

 (Fig. VII.). With the mouth went 

 the hydrocoel. The attachment ap- 

 pears to have been towards the right 

 side, for thus only can we account for 

 the fact that the structures on the 

 left of the Dipleurula increased at the 

 expense of those on the right. It 

 was therefore the left hydrocoel (l.hc) 

 and stone canal (s.c) that moved 

 upwards with the mouth, while those 

 on the right disappeared. The 

 nervous structures of the anterior 

 end remained there or, possibly, 

 atrophied. The forward portion of the anterior coelom (a.c) shared 

 in the construction and elongation of this region ; but its hinder 

 portion was dragged up along with the hydropore (M ) and formed 

 the " parietal canal " (par), so called because it lies along the outer 

 wall of the larva. The left posterior coelom (l.p.c) of the Dipleurula 

 was caught in between the oesophagus and the stomach, and so 

 passed upwards, towards what we may now call the oral pole of 

 the fixed stage ; while the right posterior coelom (r.p.c) was pushed 

 downwards by the stomach pressing to the right and thus came 

 to lie nearer the aboral pole. The blastopore is early closed in 

 the ontogeny of Antedon, but we infer from the position of the 

 larval rectum that in phylogeny the anus (As) did not move 

 upwards so rapidly as the mouth. The effect of these changes 

 was a torsion of all the structures in the upper part of the 

 body. The gut was thrown not into a simple loop, but into a 

 dextral coil. The pressure of the oesophagus against the hydrocoel 



FIG. VII. 



Diagrammatic reconstruction of the 

 imagined primitive Pelmatozoic ances- 

 tor. Compare carefully with Fig. I. 

 ami with adjoining text'. 



