222 ECHINODERMATA. 



4. The modification of the larva into the adult Echinoderm. 



This history is so intricate and so difficult to understand without 

 models, that I prefer to restate the general fact that the development is 

 indirect, that the adult is a new formation within the larva, retaining 

 the water-vascular system and mid-gut, but absorbing or rejecting the 

 provisional larval structures. It is striking that in spite of the final 

 differences, the first steps in the upbuilding of the adult, and especially 

 of its skeleton, are closely parallel in the five classes. 



One of the most important changes is that from bilateral to radial 

 symmetry, in connection with which it is very likely that the primitive 

 ancestor was bilaterally symmetrical, and that the radiate symmetry was 

 acquired by early sessile or sedentary Echinoderms, such as the Cystoids. • 



It is very difficult to compare the Echinoderm larvae, even in their 

 simplest form, with those other animals. The nearest type is perhaps 

 the Tomaria of Balanoglossus, but it again is very unique. One 

 naturally tries to compare the Echinoderm larva with the Trochosphere 

 of Annehds, but the differences are very marked. 



Pedigree and Relationships of Echinoderms. 

 I shall not attempt to draw any genealogical tree, for that is apt to 

 give an unwarrantable appearance of precision to merely tentative 

 genealogies. The advanced student should read the remarkable chapter 

 on Echinoderms in Neuraayr's Stamme des Thierreichs. I think that - 

 the following conclusions are sufficiently cautious : — 



(1) The Echinodermata do not, seem to give origin to any other 

 type, in the way for instance that " reptiles " evolved into birds, but it 

 is possible that an offshoot from near the base of the Echinoderm branch 

 was not far removed from the Balanoglossus twig, and was thus connected 

 to Vertebrates. 



(2) It is likely for many reasons that the Echinoderms arose from 

 some worm type, to which the Holothurians retain more outward re- 

 semblance than do the others. 



(3) From anatomical, embryological, and palasontological reasons, it 

 is certain that the Holothurians, Echinoids, Asteroids, and Ophiuroids 

 have diverged in a direction different from that along which Crinoids 

 have evolved. Moreover, the Crinoids present many affinities with 

 Blastoids and Cystoids. 



(4) The Cystoids form a central class, and the general relationships 

 may be represented thus (with one alteration after Neumayr). 



Ophiuroidea — Asteroidea^^^ . ^^Crinoidea 



Echinoidea^ Cystoidea <^Blastoidea 

 Holothuroidea (central) 



