4 BRITISH PALEOZOIC ASTEROZOA. 



The body sank in over tlie radial canals, and grooves were found converging on 

 the month. The cilia became confined to the sunken groove (and to the podia). 



Until recently the ordinary concept of a Pelmatozoon was that suggested by its 

 name, " a form fixed by a stalk." Kirk (36) and Bather (9a) have recently shown 

 that this is a mistake, for a large number of the forms which feed in this way lead 

 a free or semi-free existence. Thus Rhipidoci/sfis, as Bather remarks (9a, p. 503), 

 " like little wanton boys that swim on bladders," floated in the water, supported 

 by its swollen root-sacks; again, 'Ereimocrlnus had arms shaped for paddling. 

 (Kirk 36, p. 124). No Pelmatozoon, however, used its tube-feet for walking. 



i!. Tlie Eleutherozoa. — This group consists of Starfishes, Brittle-stars, Sea- 

 urchins and Sea-cucumbers, all of which use their tube-feet for progression. The 

 geological record is neither old enough nor sufficiently complete for us to trace 

 the beginnings of the Eleutherozoa, and in consequence only suggestions can 

 be advanced as to the manner in which they arose from the primitive fixed form. 

 At present two theories need consideration : 



(i) The view held by Bather that the Eleutherozoa descended from the small 

 disc-shaped sessile Edrioasteroidea. " Some of them were permanently fixed to 

 the sea-floor, but others acquired a flexible under-surface surrounded by a rigid 

 frame of larger plates, and they held on to the sea-floor in limpet-fashion. Some 

 of this latter type might occasionally be washed loose by the waves, and over- 

 turned so as to bring some of their food-grooves in contact with the ground. 

 Those which had stronger podia took advantage of these new conditions, and 

 their podia became transformed into locomotor organs with suckers at the tips. 

 At the same time the ciliated grooves ceased their task of sweeping food-currents 

 to the mouth, in proportion as the animal Avas able to move its body and mouth 

 from one source of food to another" (4, p. 12). 



Bather further supposes that the Holothurians (Sea-cucumbers), Echinoids 

 (Sea-urchins) and Asteroidea came off from tlie Edrioasteroid stock at different 

 periods. 



The advantage of this suggestion is that the Edrioasteroidea are known forms, 

 to the structure of which we can refer, and that the arrangement of the ossicles 

 bordering the groove is very similar to that found at any rate in the Asteroidea 

 {vide infra). 



(ii) The view of MacBride that the Eleutherozoa are descended from a fixed 

 form which had a long flexible stalk. The upper portion of the body became 

 flexed on the stalk, and the creature took to " grabbing " with its tube-feet at 

 the food drifting along at the bottom of the sea. The podia obtained opportunity 

 of use as " walking " organs, and the stalk became of less and less use as an 

 anchor. MacBride (43) supposes that the first Eleutherozoa were asteroid-like. 

 The remaining Eleutherozoa descended from these early " asteroid " forms. 



I have attempted to reconstruct, independently, the fixed ancestor of tlie 



