304 THE AMEBIC AX NATURALIST [Vol. XLVIII 



canal. Excepting the echinoids and crinoids in which 

 there is either no distinct Polian vesicle or else a simple 

 glandular structure, those echinoderms that have retained 

 the most distinctive type of radial structures have also 

 as a rule, retained the most symmetrical arrangement of 

 the Polian vesicles. Presumably these forms, the aster- 

 oids and ophiuroids, have quite recently abandoned the 

 fixed stage, and each individual usually has four Polian 

 vesicles and a stone canal, one in each interradius. 

 Among most of the holothurians a secondary bilateral 

 symmetry has become superimposed over the radial type, 

 and it is reasonable to suppose that there was a time in 

 the ancestral history of Thyone when the Polian vesicles 

 were symmetrically and radially disposed, or else the 

 animal quit its fixed habits before the radial symmetry 

 of the vesicles was thoroughly established. In the one 

 case we would have a regression, a sort of backward 

 retracing of the steps of evolution, or, which seems more 

 probable, the ancestors of Thyone began a free-living 

 existence before the radial arrangement of the Polian 

 vesicles had become complete. Also the fact that the 

 embryology of the holothurian egg is probably much 

 compressed and shows no trace of a fixed stage indicates 

 that the corresponding ancestral stage was compara- 

 tively short, or, very remote. Since the modern habits of 

 Thyone are bilateral, and since it is altogether improb- 

 able that such habits would produce the present arrange- 

 ment of Polian vesicles, the position of these organs must 

 be due to ancestral influence. 



Now the Polian vesicles are capable of contracting and 

 expanding and their function when they are well devel- 

 oped is to act as accessory reservoirs of the water-vas- 

 cular fluid. Muscle and connective tissue in the wall of 

 the vesicle furnish the means to do this work. Of course, 

 if the ampullae are well developed there is little or no 

 need of Polian vesicles, as is the case in Asterias. But, 

 though the size and number of these vesicles is function- 

 ally correlated with the general development of the 

 water-vascular system, especially of the oral tentacles, 



