170 



Snatangoida) living in the Atlantic Ocean and having an egg diameter of 3.2 

 mm (Thorson, 1936). On the other hand, sea urchins with large eggs and 

 lecithotrophic larvae occur in the tropical zone of the Pacific Ocean. They 

 belong to the following three orders: Echinothurioida — Asthenosoma ijimai; 

 Cidaroida — Heliocidaris erythrogramma, and Clypeasteroida — Peronella 

 japonica. The development of these three species has been fairly well studied. 



In Peronella japonica (Mortensen, 1921; Okazaki and Dan, 1954) the egg 

 diameter is 300 |im. Already in the late gastrula stage, invagination appears 

 in its anterior end which enlarged to form the amniotic sac between the 

 archenteron and ectoerm. In the pluteus the arms are sometimes not at all 

 developed, but usually one pair of arms may be present. After 60 hrs, the 

 amniotic sac in which development of the definitive individual takes place is 

 everted and the young urchin begins to move on the substrate by means of 

 the ambulacral podia. Remains of the larval body are retained for some time 

 (Figure 126). 



In Heliocidaris erythrogramma (Mortensen, 1921) the egg diameter is 

 500 |im. About 42 hrs after fertilization, the amniotic invagination appears, 

 which remains open. It lies in the posterior part of the larva, is covered with 

 cilia, and occupies three-fourths its circumference. On the fourth day of 

 development, the oral disk of the definitive individual appears on the outer 

 surface and the ends of the larval body bend to the aboral side. The devel- 

 oping young urchin feeds on the yolk present in the larval body. According 

 to the data of Mortensen, the development of this urchin takes two days 

 (Figure 127). 



The largest eggs, 1 .2 mm in diameter, have been described in Asthenosoma 

 ijimai (Amemiya and Tsuchiya, 1979). The blastula hatches after 27 hrs. The 

 gastrula is formed after 48 hrs. By the third day of development, the late 

 gastrula has become flattened in a dorsoventral plane. After 4—5 days, two 

 tubercles appear on the ventral side of the larva. At this stage, the larva 

 remotely resembles the early bipinnaria of the sea stars. Four strongly reduced 

 arms appear in the larva after five days, which are devoid of rods and differ 

 in position from the arms in planktotrophic larvae (Figure 128). At this time, 

 the rudiments of organs of the juvenile also appear but the amniotic sac does 

 not develop. By the end of the fourth week, metamorphosis has been com- 

 pleted and the juvenile settles on the substrate (Figure 128). 



IDENTIFICATION OF PELAGIC LARVAE OF SEA URCHINS 

 (Terminology and Diagnostic Characters) 



The larvae of sea urchins have an elongate body that may taper in the basal 

 part. At the level of the esophagus, several pairs of processes are seen, the 

 so-called arms. The characteristic feature of the larvae of sea urchins is the 



