1915 397 F. A. Bather — Studies in Edrioasteroidea. 



In Pelmatozoa also the quinary axis conies almost to coincide with 

 the binary axis, but its direction is reversed. 



Of these changes the Pelmatozoic Theory offers an explanation 

 which, whether correct or no, is at any rate less transcendental than 

 appeals to "idiosyncrasy", invisible " Yererbungspotenzen ", or 

 " hemiplexie teratologique By this theory the relations of the 

 axes of symmetry emphasized by Herouard are interpreted in 

 accordance with the life -history. The Dipleurula becoming fixed by 

 its anterior end, the mouth passes to the opposite pole, and thus arise 

 the Pelmatozoa with axis vertical and in a reversed direction to that 

 of the Dipleurula. The change from these to the earlier Eleutherozoa 

 is complicated by the retention of the stalk ; the whole oral surface 

 bends over, and the quinary axis is at right angles to that of the 

 Dipleurula. In the later Eleutherozoa the process is carried further, 

 and the quinary axis returns almost to the position of the original 

 binary axis. 



The associated changes in the torsion of the gut and in the 

 movements of the coelomic cavities, loosely moored in the body-fluids, 

 are described in the Treatise on Zoology (1900), and in the writings 

 previously referred to (1901, 1902, 1911). We pass them by for the 

 moment to consider the second morphological feature, the radiate 

 symmetry. 



lladiate symmetry arises and is maintained when an organism is 

 evenly related to its environment on all sides. This happens in the 

 case of fixed organisms, when they gather their food from all quarters 

 irrespectively, by leaves or by tentacles or by ciliated grooves ; in 

 the case of free organisms, when they move indifferently in the 

 direction of any radius. This evenness of relation may be interfered 

 with from without or from within : from without, especially in the 

 case of a fixed organism, when the situation is such that currents of 

 air or water, or rays of light, beat on it mainly from one side ; from 

 within, by some change in the organism itself whereby its relation to 

 the environment is altered, as when a free organism takes to moving 

 in the direction of a particular radius (cephalization), or when a fixed 

 organism for some special purpose (e.g. excretion) enlarges one of its 

 unpaired asymmetrically situate organs. The Echinoderma, which, 

 even in the most specialized Ophiuroid, never have attained an absolute 

 radiate symmetry, present us with examples of almost every con- 

 ceivable departure from such symmetry in forms both free and fixed. 



Now Messrs. Gemmill and MacBride fully accept these views as 

 to the origin of radiate symmetry, and believe with me that the 

 free Echinoderms owe the radiate arrangement of their organs to 

 inheritance from a fixed ancestor, which acquired that arrangement 

 in consequence of the radiate extension of its food-catching organs. 

 But the mode of fixation which they postulate, namely by a stalk 

 asymmetrically placed on the oral face, is not one calculated to lead 

 to such symmetry. Not merely does it appear exceedingly improbable 

 that an animal feeding solely by ciliated grooves should ever have 

 assumed or maintained such a position, but even if it did so it would 

 not have been evenly related to the environment on all sides. In those 

 Pelmatozoa that have assumed a similar position with reference to 



