CILIATION OF ASTERIDS. 17 



ciliary feeding still occurs in an adult starfish, we have a hint 

 of the manner in which nutritional continuity was maintained. 

 The fixed ancestor fed by ciliary activity during the time 

 when it was becoming, and at first after it had become, an 

 Asterid. It is not necessary that an attached ciliary-feeding 

 organism should have its food-collecting area looking away from 

 the surface of attachment. Given an aboral skeleton, the 

 advantage will be the other way so far as protection from enemies 

 is concerned. The softer oral field could be closely appressed 

 when necessary to the surface of attachment through the agency 

 of the adhesive tentacles, which became evolved into the sucker- 

 foot system. " Easing up " from the surface (cf . Crania) would 

 allow ciliary feeding at any time. The tentacles might also 

 begin to capture drifted or moving prey, as MacBride suggested. 

 But in any case gradual perfection of the adhesive function of 

 the tentacles would allow atrophy and separation of the attaching 

 stalk, and, with freedom of movement now possible, opportunities 

 for the capture of the larger kinds of food would be vastly 

 increased and would be utilised sooner or later. Ciliary nutrition 

 would thus be first supplemented and afterwards completely 

 (mast Starfish), or partially (Porania), replaced. It is worthy of 

 note in this connection that the larval histoiy of Porania, so 

 far as it is known, exhibits all the primitive characters referred 

 to above in connection with the development of Asterias rubens. 



Summary. 



A. Ciliary Action. — The arrangement of the ciliary currents on 

 the various surfaces of four widely different species of Starfishes 

 has been described in detail. This arrangement is constant for 

 all individuals in each of the species, and, except as regards 

 external surfaces, is practically the same in all the species. 

 Everywhere the arrangement has been shown to be explicable 

 by physiological needs. Ciliation in the perihaemal spaces has 

 been demonstrated. 



B. Ciliary Feeding. — In the case of Porania pulmlhis a 

 mechanism for ciliary feeding has been shown to exist, and the 

 results of experiment demonstrate that this kind of feeding 

 actually takes place. As regards Astropecten, it is only shown, 

 so far, that the arrangement of the actinal and abactinal cilia 

 makes ciliary feeding possible. In Solaster papposus ciliary 

 feeding probably takes place, but in an entirely minor degree. 

 The other Starfishes examined gave negative results. The- 

 important bearing of the above results on questions of phylo- 

 geny is briefly discussed. 



References. 



1. Bather, F. A. — Art. " Echinoderma," Encycl. Brit, 11th ed, 



vol. viii. p. 875. 

 Proc.Zool.Soc— 1915, No. II. 2 



