eoactinid.f;. 197 



Since I wrote the previous pages ray attention has been called to the important 

 work of Petersen and his colleagues (104), which, by its exact information on the 

 feeding habits of recent forms, enables us to judge with more certainty as to 

 the mode of life of their fossil relatives. In particular it helps us to understand 

 the differentiation which has arisen among the forms we are now considering. It 

 always seemed surprising to me that, in the older Palaeozoic rocks, the true Asteroid 

 forms (carnivorous) were relatively few in number, and not so much differentiated 

 as the forms which were either Ophiuroidea or transitional towards the Ophiuroidea 

 as judged by the manner of growth of the interradial areas — a feature associated 

 (see above, p. 195) with the mud-eating habit. Petersen's observations show that, 

 even to-day, the mud-eating habit is the general rule among the lower animals 

 which live on the sea-bottom, and that it is plant detritus in the uppermost layer of 

 mud which provides the nourishment. The plant detritus in Danish waters is 

 formed from grass-wrack (Zostera) and its attendant microflora living under the 

 water. This plant produces " in leaves alone 8,232,000 kg. annually in the Danish 

 waters east of the Skaw. In course of time these plants of the benthos formation 

 die, or are torn away by the currents or action of the waves, and carried out, in 

 whole pieces or fine particles, into the deeper water, where they are deposited on 

 the bottom in the form of detritus" (1914, p. 46). Here they form an uppermost 

 brown layer of the mud, " especially well suited as nourishment for the animal life 

 of the sea-bottom." The animals which feed on the plant detritus in the brown mud 

 may live either buried in the bottom or not buried in the bottom. 



This is well illustrated by the following picture of the mode of life of the 

 Echinodermata given 1914, pp. 61 — 64: 



(1) Those without arms, and living buried in the bottom. 



Echinoidea irregulaeia without masticatory organs (Atelostomata) : 

 Brissopsis lyrifera, Forb. Echinocardium flavescens, Mull. 



Echivocardium cordatum, Pennant. Spatangus purpureas, Midi. 



" All these are typical detritus-eaters, feeding directly on the upper layer of the 

 bottom, without any previous sorting of the detritus. As the aquarium observations 

 have shown, they live buried in the bottom, maintaining communication with the 

 surface by means of vertical tubes, and drawing clown their food by means of the 

 far-reaching ambulacral feet . . . the Echinocardium cordatum frequently 

 devours, together with the bottom detritus, numbers of quite small young 

 bivalves . . ." 



(2) Very mobile arms; living buried in the bottom. 

 Amphiura chiajei, Fori). Amphiura filiformis, Mull. 



"The Amphiura species live, as I have frequently observed in aquaria, buried 



27 



