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APPENDIX 4. EMBRYONIC STAGES IN THE DEVELOPMENT OF TROCHUS NILOTICUS OBSERVED UNDER A MICROSCOPE

Most marine snails have the same morphology until the veliger stage when shell sculpture can be clearly distinguished. Green snails show striations across the shell, whereas the trochus shows longitudinal striations.

(1) Fertilised egg as seen under the microscope. Two poles are also seen, by 2 small bubbles within the viteline minor.
(2) Fertilised egg undergoing mitosis, 2 cells as seen in this photo.
(3) Green Snail dividing into 4 cells
(4) Trocophore just before hatching. Velum starting to grow.
(5) Green snail veliger before torsion (pre-torsion stage) with embryonic shell growth.
(6) Trochus veliger before torsion (pre-torsion stage) with embryonic shell growth. Note shell sculpture differs from green snail.
(7) Veliger with complete shell, after a 180° torsion. Operculum together with the developing foot was observed.
(8) T he foot was seen developing and the veliger retracted into the shell when it was disturbed.
(9) Pediveliger (Creeping stage). The eyes were seen as black dots and the cephalic tentacles were beginning to grow.
(10) Juvenile seven days after settlement. The snail uses its foot for mobility.

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