52 THE AMERICAN ARBACIA 



e. Breeding Season; Shedding of Eggs and Sperm 



With few exceptions, the unfertilized eggs and the sperm of sea urchins 

 are shed in the sea water; the eggs are there fertilized and develop into 

 free-swimming larvae or plutei which pass through a complicated meta- 

 morphosis before acquiring the adult form. This is true of Arbacia 

 punctulata. However, a few viviparous species of Echinoidea have been 

 described, mostly from the southern hemisphere. The eggs which arc 

 usually very large, 1-2 mm. (Hesse u, Doflein, 1914, H p. 619; Tab. 

 Biol. VI, p. 503), develop in a sort of brood pouch formed by the long 

 spines, either dorsally or ventrally. The larval stage is usually imper- 

 fectly developed or omitted entirely and the animals come out as 

 young adults. Among these may be mentioned : Anochanus sinensis from 

 the China Sea, as described by Grube (1868); Hemiaster (Abatus) cor- 

 datus, collected on the Transit of Venus expedition (1874-1875) by 

 Dr. Jerome H. Kidder (1875- 1876), near the Kerguelen Islands and 

 described by A. Agassiz (1876) ; the same species and Cidaris nutrix and 

 Goniocidaris canaliculata, collected on the Challenger expedition (1873- 

 1876) near the Kerguelen Islands and the Falkland Islands and de- 

 scribed by Wyville Thomson (1877, 1878). One of his interesting 

 woodcuts is reproduced as Fig. 8 in this monograph; several of them 

 are reproduced in MacBride's text-book (1906, p. 535, 555, 603). An 

 account of these and other Echinoidea collected on the Challenger Expe- 

 dition may be found in A. Agassiz's report (1881). Hypsiechinus corona- 

 tus and Goniocidaris umbraculum and other species are described by 

 Mortensen (1927a, p. 293; 1927b; 1931, p. 7). 



The breeding season for Arbac\a punctulata at Woods Hole is from 

 the middle of June till the middle of August, though the season varies 

 from year to year according to temperature. During May and early 

 June, the gonads are small and it is often difficult to tell whether they 

 are ovaries or testes. The males ripen earlier and remain in good con- 

 dition later than the females. The eggs mature in the ovary, so that 

 ^vhen they are shed or procured from the ovary during the season most 

 of them have already given off (and lost) their polar bodies, and the 

 eggs are ready for fertilization and immediate development. For about 

 a week before the middle of June, the eggs are ripening and all sizes 

 of eggs in the germinal vesicle stage and all stages of polar body for- 

 mation are then found. Some immature eggs are often found among 

 the ripe eggs later on until towards the end of the season. There are 

 usually many immature eggs late in the season in animals from the 

 new beds at Menemsha Bight and Fisher's Island. 



