264 PHYLUM ECHINODERMA. 



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 larva.-, 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. 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 Antcdon is regarded by many naturalists as a point of much 

 importance. 



RELATIONSHIPS OF ECHINODERMA. 



The Echinoderms form an exceedingly well-defined phylum, but 

 the Holothurians especially show how many of the significant char- 

 acters may be lost. In that class 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 dis- 

 tinctively 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. 



It is conceivable that Holothurians of the worm-like Synapta type 

 are nearest the primitive stock of Echinoderma. But there are stronger 

 arguments in favour of the view that the free forms, the Eleutherozoa, 

 have been derived from attached Pelmatozoic ancestors. The extinct 

 Edrioasteroidea are in some ways intermediate between the Cystidea 

 and the Eleutherozoa. 



