250 EQHINODERMA. 



temporary calcareous supports. This quaint form is often compared to 

 a six -legged easel. 



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



This history is so intricate and so difficult to understand without 

 models, that it may be better simply to state 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. As certain parts are broken down, others 

 are built up, chiefly through the agency of the wandering amoeboid cells 

 of the mesenchyme. The first steps in the upbuilding of the adult, and 

 especially of its skeleton, are to some extent parallel in the five classes. 



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

 symmetry. In connection with this, it has been conjectured that the 

 primitive ancestor was bilaterally symmetrical, and that the radiate 

 symmetry was acquired by early sessile or sedentary Echinoderms, such 

 as the Cystoids. As we have already seen, the adults in the different 

 classes tend to acquire an independent and secondary bilateral symmetry. 



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

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

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

 naturally tries to compare the Echinoderm larva with the Trochosphere 

 of Annelids, but the differences are very marked. 



Pedigree and Relationships of Echinoderms. 



Concerning the exact relationships of the different classes of Echino- 

 derma, there is still considerable doubt. The following account is 

 based upon the views set forth by Professor Jeffrey Bell, but the 

 student will do well to realise that in this, as in most problems of 

 phylogeny, there is little certainty. 



The Holothurians have no aboral system of plates, and the radial 

 symmetry does not effect the reproductive organs. These two negative 

 characters, combined with some positive ones, may indicate that the 

 Holothurians are primitive, and, as is certainly suggested by their 

 external appearance, have affinities with the supposed " worm-like " 

 ancestors of Echinoderms. 



Again, some members of the heterogenous class of Cystoids are 

 extremely primitive, but differ from the Holothurians in the possession 

 of an aboral system of plates, alternately radial and interradial. From 

 this primitive Cystoidean stock, two branches diverge. The one leads > 

 to the sessile Cystoids, Blastoids, and Crinoids (Pelmatozoa), the other 

 to the free Echinoidea, Asteroidea, and Ophiuroidea. Of these the 

 existing Asteroidea and Ophiuroidea are late divergences from a common 

 stock. 



[TABLE. 



