222 TOWARDS THE HUMAN FORM 



to their mouths by the currents set up by the vibratile 

 cilia on their branchial groove. Theory indicates that 

 this branch must have begun with more or less spheroidal 

 forms, derived from some short worm rolled up in such 

 a manner as to describe a complete corkscrew turn. The 

 Cystids, limited to Primary times, seem to correspond 

 to this earliest phase in the evolution of the Echinoderms. 

 We have seen how embryogeny thereafter yields an initial 

 radiate form, originally without arms, from which it is easy 

 to derive all the other classes. The Cystids antedate the 

 embryonic form in question ; they seem to have been 

 affected by all manner of external influences, from which 

 they were unable to escape, owing to their fixed position. This 

 fixation has brought with it changes in form that were calculated 

 to render the initial type unrecognizable. These varied both 

 according to the age at which the embryo became fixed and 

 to the conditions of fixation. An average embryo is generally 

 fixed by means of its anterior extremity ; but these 

 particular embryos were weighed down by the lime in their 

 tissues and sank upon the ground, to which, therefore, they 

 could attach any part of their body whatsoever. Once this 

 fixation was accomplished, the embryo must have effected 

 a rotational metamorphosis, like all fixed embryos, with the 

 object of placing mouth and anus as far as possible from the 

 point of fixation. This metamorphosis was achieved by the 

 Cirripedes among Arthropods and by the Crinoids among existing 

 Echinoderms, as well as by the Tunicates, and it completely 

 altered the shape of their bodies. The Cystids evidently 

 underwent this rotation, since mouth, anus, and genital 

 orifice are usually grouped together at the opposite pole to 

 the point of fixation. Such a change could not have occurred 

 without profoundly altering the initial type, and this, no 

 doubt, is the reason why these organisms appear to be so 

 aberrant. How they obtained their food, fixed as they were 

 with scarcely any mechanism for directing alimentary or 

 respiratory currents to their mouths, remains a problem, in 

 spite of the richness of the surrounding waters in plankton. 

 It may be that their digestive tubes were highly ciliated or 

 that they fell back on a symbiosis with green Algae. 



From the Silurian onwards other classes of Echinoderms 

 are already characterized, and represented by numerous 



