244 ECH1N0DERMA. 



stage, in which three warty arms are formed at the anterior dorsal end, 

 independently of the ciliated bands. 



Ophiuroids and Echinoids. — In the Phiteus larvae (Fig. 98) char- 

 acteristic of these classes the pre-oral region remains small, while the 

 post-anal region becomes large. There is one undulating ciliated 

 band, the course of which is much modified by the growth of six long 

 arms, with temporary calcareous supports. This quaint form is often 

 compared to a six-legged easel. 



The development of these larval forms into the adult is very intricate. 

 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 larvae, even in their 

 simplest form, with those of 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 Annelids, but the differences are very marked. One of the most 

 marked of these is the absence of the apical sense organ, so charac- 

 teristic of the Trochosphere. The fact that this is represented in the 

 larva of Antedon is regarded by many naturalists as a point of much 

 importance. 



Relationships of Echinoderma. 



The Echinoderms form an exceedingly interesting class. Well- 

 defined as they are, the Holothurians especially show how many of the 

 significant characters may be lost. In that group we see how the 

 power of forming a calcareous skeleton, the characteristic tube-feet, 

 and the greater part of the peculiar water vascular system, may all 

 disappear ; it is conceivable that further modification of the same kind 

 might eliminate all the distinctively Echinoderm characters, and 

 produce an organism whose systematic position would be very difficult 

 to determine. This is important, because, as we have already seen, there 

 are many ' ' worm-like " types of whose affinities we know nothing. 

 That some of these are related to Echinoderms has been tentatively 

 suggested by many observers, and there is every reason to believe that 

 the progress of research will remove the Echinoderms from their present 

 isolated position. 



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 : — 



