Embryonic and larval development of the peanut worm Phascolosoma agassizii (Keferstein 1867) from the Sea of Japan (Sipuncula, Phascolosomatidea)статья
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Дата последнего поиска статьи во внешних источниках: 18 июля 2013 г.
Аннотация:All stages of the embryonic and larval development of Phascolosoma agassizii from Peter the Great Bay (Sea of Japan) were studied and illustrated using light and electron microscopy. The eggs of P. agassizii have the form of an ellipsoid (long and short axes about 100 and 70 mm, respectively). Egg cleavage is typical, spiral, and unequal. Gastrulation occurs by epiboly. This species possesses two pelagic larval stages, a lecithotrophic trochophore and a planktotrophic pelagosphera. The transformation of trochophore into pelagosphera occurs 80–90 h after fertilization. After 120–180 h, the larva has developed all systems of organs characteristic of the pelagosphera and is capable of feeding. At day 10, pelagospheras can settle, for some time, on the aquarium bottom and move on a
ciliated lip, collecting food with the aid of a buccal organ. In addition, the larvae periodically attach themselves to the aquarium bottom or to the surface film of the water by means of a terminal organ. The trunk of the larva elongates by enlargement of the region behind the dorsal anal opening, which is located almost in the middle of the trunk region in the 15-day old larva. In the laboratory, 1-month old larvae spend the greater part of time in the attached state. Being attached by a glandular terminal organ to the aquarium bottom, they characteristically bend the body, actively feeding on microalgae from the substratum surface. The differences in the development of P. agassizii in the isolated West-Pacific and East-Pacific populations are shown and discussed.