44 THE AMERICAN ARBACIA 



Clark, 1 921; Lindahl and Runnstrom, 1929, p. 407; Mortensen, 1943, 

 M III, 3 p. 134). 



"A number of specimens ( Psammechinus miliaris) were held some time 

 in a basin with running water, alternately kept in light or in total dark- 

 ness. When in the light, the specimens covered themselves with pieces 

 of algae, etc., when in the darkness they dropped these covering 

 pieces" (Mortensen, 1943, M III, 3 p. 135). Several explanations for 

 the habit other than light have been offered: concealment against 

 enemies, camouflage, protection against drying in low water (Orton, 

 1929). Boone (1928) thought it must be to prevent detection by ene- 

 mies or potential victims rather than protection against light, for he 

 found in the case oi Lytechinus that specimens kept in the laboratory at 

 Miami, Fla. in relatively dark aquaria "for weeks at a time camou- 

 flaged themselves quite as thoroughly and industriously as their rela- 

 tives on the open reefs" (p. 21). This has been criticised by Mortensen 

 (1943, M III 2, p. 443). 



The best known species having the habit of decorating themselves 

 are Lytechinus variegatus (H. L. Clark, 192 1; Boone, 1928; Mortensen, 

 1943 M III 2, p. 442) ; Psammechinus miliaris (Lindahl and Runnstrom, 

 1929; Orton, 1929; Mortensen, 1943 M III 3, p. 134); Psammechinus 

 microtuberculatus (Noll, 1881); Paracentrotus lividus (Mortensen, 1927, p. 

 308; 1943, M III 3, p. 164); Sphaerechinus granulans ( Toxopneustes brevi- 

 spinosus) (Dohrn, 1875; Mortensen, 1943 M III 2, p. 522); Tripneustes 

 esculentus (H. L. Clark, 192 1), and the Japanese species Strongylocen- 

 trotus pulcherrimus (Dan, personal communication). 



The ancients had a curious explanation for this habit, given by 

 Pliny and referred to by many of the older writers, even as late as 

 Camerarius in 1654 in his book on symbols which has an interesting 

 picture of an urchin covered with stones (Fig. 3). They thought that 

 at the approach of a storm the sea urchins cover themselves with small 

 stones to provide a sort of ballast to prevent them rolling around and 

 wearing away their spines. When the urchins were covered with stones 

 it was an indication to sailors that a storm was approaching and they 

 should anchor their ships (see under Historical). 



f. Geotropism 



No data have been found for geotropism in Arbacia, punctulata or lixula, 

 but Diadema and Lytechinus are negatively geotropic, climbing up- 

 wards in an aquarium, irrespective of dark and light and also of access 

 to oxygen (Parker, 1922, 1936). Other urchins found to be negatively 



