I« THE AMERICAN ARBACIA 



denen jede noch ein FuBgestell von 2 Staben abgiebt... Da einmal 

 alles einen Namen haben muB, so mag dieser Korper Pluteus heiBen, 

 was so viel als Staffelei oder Gestell bedeutet" (1846a, p, 108, no). 

 The Latin word, pluteus, however, is according to both Latin-German 

 and Latin-EngHsh dictionaries usually applied to a military structure, 

 shed or shelter {{vom. pluvia, rain), or to a stand or bookcase. But, of 

 course, the shape of a shelter might have suggested an easel to Miiller. 

 The English word, easel, comes from the Latin asellus, or little ass, 

 which carries things. 



Johannes Miiller published many papers on the plutei of different 

 species of Echinoderms and their metamorphosis between 1846 and 

 1855, including a series of nine in the Abhandlungen der Koniglichen 

 Akademie der Wissenschaften zu Berlin ; these are classics and the illustra- 

 tions are particularly fine. A very good list of studies on larval forms 

 arranged by species, up to 1921 is given by Mortensen (1921, p. 12, 

 and a list arranged phylogenetically by Fell, 1948). Among the most 

 complete studies are those oi Arbacia punctulata by Brooks (1882) and 

 E. B. Harvey (1949); A. lixula and Paracentrotus lividus by von Ubisch 

 (1913a, b) ; Echinocyamus pusillus by Theel (1892); Echinus esculentus by 

 MacBride (1903, 1914a. Text-book of Embryology vol. i, p. 504, and by 

 Shearer, De Morgan and Fuchs (1914); Enchinocardium cor datum, by 

 MacBride (1914b); Heliocidaris crassispina, Mespilia globulus, Strongy- 

 locentrotus pulcherrimus and other Japanese forms by Onoda (1931, 1936). 

 Especially notable are the Studies of the Development and Larval Forms of 

 Echinoderms hy Mortensen (1921; 1931, I and II; 1937, HI; 1938, IV). 

 This last series (I-IV) comprises studies made during expeditions to 

 Kei Islands (Amboina), Java-S. Africa (Onrust, Mauritius) and Egypt 

 (Ghardaqa). See p. 117. 



The pluteus of Arbacia is somewhat different from that of other sea 

 urchins. The development of the three or four day pluteus of the Euro- 

 pean species, A. lixula was described by Busch in 1849, and later stages 

 by Johannes Miiller in 1854 under the name of Echinocidaris aequituber- 

 culatus; he figures the six-armed stage, which has been copied in 

 several textbooks, and the later stages and metamorphosis. A more 

 complete study of A. lixula has been made by von Ubisch (1913a, b, c, 

 1927, 1932). The late pluteus of A. stellata (incisa) from the Gulf of 

 Panama, which is similar to that of ^. punctulata and A. lixula, has been 

 described and figured by Mortensen (1921 p. 29, Plate VII, Fig. 3). 



The development of Arbacia punctulata, the pluteus and metamor- 

 phosis, is quite similar to that of ^. lixula (see Chapter 15 and Plate VI 

 of this Monograph). The pluteus was first raised by A. Agassiz at his 



