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XVII.—Embryogenesis and Larval Organs of the Giant Land Snail Achatina fulica Bowdich
Published online by Cambridge University Press: 11 June 2012
Synopsis
Organogenesis is completed by 15 days. A modified veliger stage is present. Torsion begins at about the sixth day. The foot rudiment is first to appear. The stomodæum appears as a new structure. The salivary glands develop from the stomodæum by evaginations. The œsophagus is stomodæal, while the crop and the primitive stomach are archenteric in origin. The digestive gland develops as two outgrowths of the primitive stomach. The hind gut is formed by the cells proliferated from the closed posterior end of the archenteron. It opens into the primitive stomach in a 2·5 mm. embryo, and the anus appears very late. The proctodæum is absent. The primordium of the pericardium with the heart, definitive kidney and gonad is mesodermal. The ureter is ectodermal and arises from the mantle cavity by evagination and subsequently communicates with the kidney. Lung and mantle are ectodermal and arise by separate invaginations. The lung fuses with the mantle and loses its separate existence from about a 4 mm. embryo. The nervous system is early to appear and the sense organs, except the statocysts are late in origin. They are ectodermal. The larval organs are: velum, podocyst, larval kidney, nuchal cells and larval digestive gland. The prototroch is the rudiment of the velum. It appears in a very early embryo from the posteroventral part and marks off the anterior border of the stomodæum. The cells of the prototroch become hyaline, vacuolate and develop cilia in a 2·25 mm. embryo, and it is transformed into velum. The velum helps in driving albumen into the larval and adult digestive glands and is resorbed in the body-wall epithelium. The rudiment of the foot first appears after the completion of gastrulation, and the podocyst is differentiated from it later. It steadily increases in size, assumes a hood-like shape, exhibits contraction and relaxation movements and begins to reduce when the embryo attains 3·25 mm. size. It is completely resorbed in the foot. The larval kidneys are mesodermal in origin, appear, in a 920 μ embryo. They begin to degenerate after the formation of the functional definitive kidney and disappear in a late larva. The kidneys are U-shaped, the cells of the closed anterior and bear pseudopodia, which are almost totally replaced by cilia afterwards. The kidney opens to the exterior posteriorly. The cells become vacuolated and excretory granules accumulate in the vacuoles. The nuchal cells first appear in a 2·5 mm. embryo and are excretory. They increase both in number and size with the growth of the embryo and persist throughout the larval period. The endoderm cells of the major portion of the archenteron except a small posterior part, enlarge in size, develop vacuoles and form a larval digestive organ for the purpose of digestion of albumen. Even after the formation of the adult digestive gland, the larval digestive gland functions for a short time, then degenerates and disappears at the end of the larval life.
- Type
- Research Article
- Information
- Proceedings of the Royal Society of Edinburgh, Section B: Biological Sciences , Volume 68 , Issue 3 , 1962 , pp. 237 - 260
- Copyright
- Copyright © Royal Society of Edinburgh 1962
References
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