EOACTINID^E. li> ( .> 



b. Astbboidea. Arms slightly mobile. 



Asterias rubens, L. Astropecten mulleri, Mull, et Tr. 



Solaster papposus, L. Cribrella sanguinolenta, Miill. 



Solasler endeca, L. 



"All these animals are distinctly carnivorous." (It is shown that they will live 

 on Mollusca, Gasteropoda, Crustacea, Worms, Bryozoa, Ecldnus and other 

 Echinoderma. No animal is safe from the attacks of Asterias, which also devours 

 carrion in large quantity.) 



Petersen also shows (11*18, p. 14) that "where the sea bottom is level, i.e. 

 formed of fine sand, clay or mud without foreign bodies of any considerable size, 

 the animal population is uniform throughout large tracts," which can be compared 

 with the different vegetation tracts on land such as nieadowland, moorland, culti- 

 vated fields and the like. These uniform populations he calls animal communities, 

 and suggests that it is a combination of local physical conditions with the inter- 

 play of the various organisms which determines the constitution of these com- 

 munities. Thus Echinocardium is replaced by Brissopsis as a sandy bottom passes 

 into clay, and " it is remarkable to note how in those communities where Amphitira 

 spread their arms abroad, forming a network in the bottom, extremely few 

 bivalves are found at all. The young will here doubtless as a rule be devoured 

 while still quite small by the Amphiwa, and only a few individuals of certain 

 species manage to survive " (1918, p. 17). 



It seems to me that Petersen might have carried his comparison with the land- 

 flora a stage further, and compared a section across the bottom to that of a section 

 through the soil of a copse, where each layer of soil is being made use of by the 

 plants, the deepest layers being occupied by the roots of trees, the next layers by 

 bulbs, and the surface layers by shallow-seated rhizomes or the roots of grass. I 

 have given here (Text-fig. 139) an imaginary cross-section through an Echino- 

 cardiwm-Tur ritella community, based on Petersen's observations, to show such a 

 comparison. The distinction between the animal and the vegetable communities 

 lies in this, that whereas the plants obtain their salts from their own layer of 

 soil, the animals, with one exception, all obtain their nourishment from the upper- 

 most layer of the mud. Nevertheless it is clear that the occupation of the various 

 levels of the bottom gives greater opportunity for the living together of a mass of 

 organisms, provided that the supply of food be sufficient. We may compare the 

 community with a town population living in flats, all being supplied with food 

 brought from the surrounding country area. 



Certain forms seize the detritus as it is falling to the bottom. Modiola, 

 Mytilus < j dulis (the edible mussel), so familiar on the supports of piers, have 

 this power. A fauna which consists mostly of organisms of this nature is 

 named by Petersen an Epifauna, and it is present (a) when the current 



