36 THE AMERICAN ARBACIA 



b. Growth and Age 



Very little is known of the growth and age of sea urchins, especially 

 o{ A. punctulata. From fertilization through metamorphosis o{ A. punct- 

 ulata takes (under laboratory conditions and about 23° C.) three or 

 four months, and the animal is then about i mm. in diameter (Plate VI, 

 Photo 11). The smallest sea urchin brought in from the outside by the 

 collectors July 22, 1952, was 6 mm. in diameter with spines, 3.6 mm. 

 without spines, and was 1.6 mm. high. (Plate VI, Photo 12). Smaller 

 specimens, just metamorphosed, have not been found, though a careful 

 search has been made on the stones and shells brought up from the 

 beds and in the plankton above the beds. The newly metamorphosed 

 young of most species do not seem to be common, but Mortensen 

 (1927a, p. 333) says the larvae and newly metamorphosed youngof 

 Echinocardium cordatum are sometimes found in enormous numbers in 

 some places. Arbacia mature when very young; several about i cm. in 

 diameter (without spines; 1.7 cm. with spines) had ripe functional eggs 

 or sperm, and the eggs when fertilized developed into fine plutei. One 

 that was 0.7 cm. (without spines) had ripe functional sperm, and one 

 that was only 0.6 cm. had well developed ovaries containing immature 

 eggs of all sizes, many of maximum size, and a few mature eggs, which, 

 however, did not fertilize. Several small urchins, 2 cm. in diameter 

 (without spines) brought in Aug. i, 1951, contained all mature and no 

 immature eggs. This early maturity has been observed in other forms. 

 Psammechinus miliaris has been found with ripe eggs and sperm when 

 1.2 and 1.3 cm. and even when less than i cm., when they are less than 

 a year old; and Echinus esculentus and E. acutus "when very small" 

 (Shearer, de Morgan, and Fuchs, 1914, p. 270; Orton, 1920, p. 352 

 footnote). Sphaerechinus granulans is sexually mature when 0.8 cm. in 

 diameter (Mortensen, 1927 a, p. 311). It is of interest that Aristotle 

 noticed this in his sea urchins, for in the Historia Animalium he says 

 "the eggs (ovaries) are found in the larger and smaller specimens 

 alike; for even when as yet very small they are provided with them" 

 (see under Historical). 



A. punctulata of all sizes have been brought in by the collectors, 

 ranging from 3.6 mm. to 56 mm. in diameter (without spines), but until 

 recently the small ones were found very rarely. Nothing is known as 

 to the age of the large ones formerly brought in and used for experi- 

 mental work. It seems probable that these came from old beds in 

 which the young ones could not establish themselves, or perhaps were 

 carried by currents, as plutei, elsewhere. The small Arbacia probably 



