12 



The food of sea urchins is very different in different species. 

 The only food that I actually observed our Arbacia eating, 

 was Clione sulfurea, a yellow calcisponge. The study of the 

 faeces however, will reveal us many other preys. The European 

 species. Echinus esculentus, chiefly feeds on the brown fronds 

 of Laminaria and other sea weeds (M c. Bride in : Cambridge 

 Natural Hist.) and its small inhabitants, chewing them all up 

 with its teeth. According to Scott 117) it eats sea- weeds and 

 sand. Chadwick 13) considers this species as carnivorous and 

 found Balanus-shells in their guts. If some specimens are put 

 into an aquarium, in which some Balani are present, they will 

 immediately attack them and chew them up. 



Not only such peaceful prey is taken however. The Neapo- 

 litan species, Spaerechinus and Toxopneustes, even capture such 

 alert crustaceans as Squilla mantis, as Dohrn 32) reports. 

 They cover themselves up w^ith mussels and other harmless 

 objects and move around camouflaged in that way. Suddenly 

 they attack their prey which can not escape any more. If a 

 dozen Toxopneustes were put into one bassin with a dozen 

 Squilla, it did not take more than eight or ten days for all the 

 crustaceans to disappear. The remarkable coordination appa- 

 rently present in these sluggish animals is very interesting from 

 the point of view of animal behavior and has been described 

 in detail by Dohrn. 



These crustaceans are caught by means of the ambulacral 

 feet : as soon as one of them gets hold of a prey, all others 

 are loosened. Some smaller animals are caught by the pedicel- 

 lariae, some by the spines. H. Eisig (Kosmos. 8. 1883. p. 28) 

 describes the capturing of a Uttle annelid by Echinus lividus 

 by means of its spines. 



Sponges, Gorgonidae, fishes. Crustaceans etc. are also prey 

 of some urchins (Prouho 103) for Dorocidaris papillata). Even 

 calcareous algae are sometimes digested. Echinus miharis bores 

 holes in rocks, covered with crusts of Lithothamnium poly- 

 morphum, for the sake of food, according to Hesse 58), maybe 

 for the sake of Ca and of protection against the waves, according 

 to Jordan 142). 



Quite a different food is taken by the Spatangoidea and 

 the Clypeastroidea, They live in the sand and eat them- 

 selves through it Since no representative of these groups is 

 studied in the present paper — the common sand-dollar, Echi- 

 narachneus parma, is too small for most purposes — I do not 

 consider it desirable to describe their very interesting habits 

 here ; the reader can find them described in a paper by H o r- 

 nyold 61) and in von Uexkiill's Umwelt und Innenwelt. 



Holothurians usually eat mud. It may be of some interest to 

 describe the feeding habits of our Thyone a little more in detail. 



