30 THE AMERICAN ARBACIA 



St. Andrews in New Brunswick, Canada ; and H, L. Clark ( 1 933, p. 79) 

 states that albino or partially albino individuals of Centrechinus antil- 

 larum are occasionally seen in the West Indies. Cuenot (19 12) found 

 entirely white specimens of Psammechinus miliaris at Arcachon "com- 

 parable to albinos," but he also found transitions to the usual color. 

 Deep sea forms are often, according to Koehler (1927, p. 9) extremely 

 brilliant. 



c. Morphology. Plate I 



At the aboral end of A. punctulata, surrounding the anus, there are 

 (usually) four anal and five genital plates including the large madre- 

 poric plate (Plate I and Figs. 6, 7). There may be only two or as many 

 as fourteen anal plates; there are often three or five (Jackson, 1927, 

 p. 464). Each of the five genital plates has typically one genital pore 

 through which the eggs and sperm exude; there are frequently two 

 genital pores in one plate, sometimes more, and in one case seven 

 (Jackson, 1927, p. 458). Between and below the genital plates are 

 five small ocular plates. There are considerable variations in the 

 number and arrangement of plates (Osborn, 1898, 1901; Jackson, 

 1927). At the oral end project the five teeth of Aristotle's lantern. The 

 lantern is composed of 40 calcareous pieces controlled by 60 muscles 

 (Jackson, 1927, p. 483; Reid 1950). 



The test is made up of small pentagonal plates arranged in double 

 rows, five narrow (ambulacral) and five broad (interambulacral) 

 rows. These plates are studded with tubercles which bear the spines 

 and pedicellariae. The pedicellariae ' have been described by Agassiz 

 and Clark (1908, p. 71). Tube feet project through a double row of 

 pores arranged in ten lines, one on each side of the narrow (ambula- 

 cral) row of plates. As growth occurs both by the increase in number 

 of plates and by increase in size of each plate, the total number of 

 plates depends on the size of the animal. It has been estimated that a 

 large Arbacia (4.5 cm. diameter) has about 1,000 plates and a small 

 Arbacia (2.5 cm. diameter) has about 700 plates. These figures are 

 based on measurements made by the students in the invertebrate 

 course at the Marine Biological Laboratory at Woods Hole in 1951, 



' The pedicellariae of Toxopneustes pileolus are poisonous (Fujiwara, 1935, Annot. ^ool. Jap. 

 15 : 62-69; Mortensen, Monograph III 2, p. 469). The long pointed spines of the Diademas 

 are poisonous (Mortensen, Monograph III i p. 249). See C.J. Fish and M. C. Cobb (Thayer), 

 1954, Noxious marine animals of the central and western Pacific Ocean. Research Report 36 of 

 U. S. Fish and Wildlife Service. 



