THE DEVELOPMENT OF RETUSA OBTUSA (MONTAGU) (GASTROPODA, OPISTHOBRANCHIA)

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Abstract
The development of Retusa obtusa (Montagu 1803) from uncleaved egg to hatching is described. Cleavage is spiral and holoblastic, and results in the formation of a stereoblastula. Gastrulation occurs partly by epiboly and partly by invagination. The veliger form is assumed within the egg membrane: the velar lobes and cilia are comparatively less well developed than those of planktonic veligers.The foregut is ectodermal in origin, whereas the hindgut is almost entirely endodermal. The larval midgut diverticula give rise directly to the adult digestive glands. The left diverticulum is larger than the right from the first. A coelom, of the type described by Saunders and Poole (1910) for Aplysia, is not present at any stage in the development of Retusa. Hatching occurs approximately 28 days (at 12 °C) after oviposition. Development is direct.The alimentary canal and nervous system show some evidence of torsion; but this asymmetry is present from the first appearance of these organ systems, and no torsional process per se is detectable in. development. No jaws or radula are present in Retusa. At hatching, the heart and adult kidney are well developed. The embryonic kidney degenerates. The larval musculature degenerates and plays no part in the formation of the adult musculature. The mantle cavity spreads posteriorly, so that the heart and kidney come to tie within the suprapallium.The conclusion reached is that the trend toward loss of genetic torsion in the opisthobranchia has advanced further in Retusa than in Aplysia (Saunders and Poole 1910). Hyperstrophy of the embryonic shell is attributed to deformation of the visceral mass by the enlarged left midgut diverticulum, before evagination of the shell gland has taken place. There has been no previous record of direct development for a shelled opisthobranch. Comparisons are made with R. truncatula, which has an obligatory planktonic phase in its life history. It is concluded that the acquisition of direct development by R. obtusa is connected with the small size of the adult animal and with its northerly range.

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