VII. Morphology and Bionomics. 1915 263 



dextral and sinistral halves. None the less, behind the simple 

 mechanical explanation there still obtrudes the question : Why did 

 one species place itself on a slope with its right side highest, and 

 another species with some other side highest? Or if the external 

 force was not gravity but the constant impact of a current, the 

 question merely has to be posed in another form. There must be an 

 answer to these questions, but we shall not discover it till we know 

 the predecessors of these species, and perhaps not even then. 



The Feeding of the Edrioasteroidea, as that of all other Pelmatozoa, 

 was no doubt accomplished by the subvective system of ciliated 

 grooves ; but it does not follow that ciliary currents were the only 

 means by which food was transported to the mouth. Reasons have 

 been given for supposing that pores passed from the groove, between 

 the floor-plates, to the thecal cavity in Dinocystis (1898, p. 545), in 

 Edrioaster buchianus (1900, p. 196), in E. bigsbyi (1914, p. 124), and in 

 Steganoblastus (1914, p. 198); and it has been argued, from the close 

 similarity of the skeletal structures to those of an Asteroid, that there 

 were external podia connected with internal ampullae. It is not 

 supposed that these podia were provided with suckers, and without 

 them they could not hand morsels of food along the grooves, or even 

 assist the process in the way described by Dr. H. S. Jennings for 

 Asterias forreri (1907, Pub. Univ. California, Zool., vol. iv, p. 93). 

 They could, however, have performed some sweeping or pushing 

 action, and may thus have contributed to the supply of food. 



Respiration, it is probable, was the primary function of these 

 podia. There is no sign of pores for papulae in any part of the thecal 

 wall, so that these would have been the only extensions of thin-walled 

 cavities available for the interchange of gases. The presence of 

 a distinct hydropore confirms the view that the water-vascular system 

 in these genera carried on some function of importance, whereas in 

 Agelacrinidae the apparent absence of a hydropore is correlated with 

 the absence of pores between the floor-plates. 



The view that active podia existed in Edrioaster and its like, to 

 which certain facts of structure point, is opposed by other facts of 

 structure, namely, the completeness with which the cover-plates 

 could close down over the grooves, and the position of the pores close 

 up to the hinge-line of the cover-plates. The latter fact led 

 Mr. W. K. Spencer (1904, p. 45) to cast doubt on the existence of 

 podia, and to suppose that the pores had "a respiratory function", 

 which means, apparently, that water was driven through them, by 

 ciliary action (?), into some endothecal cavities not precisely indicated. 

 Pores were supposed by him to be correlated with "firm skeletons", 

 and to be absent from forms with "imbricating plates", which would 

 " present large surfaces of membrane to the surrounding sea-water ". 

 This view, however, does not agree with the known facts; besides, 

 the objection is not insuperable. Of course the cover-plates of 

 Edrioaster were not closed when the animal was feeding, and it has 

 already been suggested that they might open obliquely so as more 

 readily to allow the podia to pass out between them (1914, p. 162). 

 In the same place attention was directed to the possible openings 

 immediately above the peripodium in Edrioaster bigsbyi effected by 



