Branchiostoma lanceolatumlarvae: Some experiments on the effect of thiouracil on metamorphosis

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Abstract
Larvae of Branchiostoma lanceolatum (Pallas), of a late pre-metamorphosis stage and characterized by a bright fluorescent green around the ilio-colonic ring, were taken in the Plymouth area plankton in August 1965.Adults were kept in 0.04% thiouracil to determine the general effect at this concentration. It was lethal over a period of weeks.The larvae were kept in the dark at a constant temperature of 10° C, individu-ally or in pairs, in 0.04%, 0.02%, 0.01%, 0.005% and 0.0025% thiouracil, in saturated solutions and suspensions of thyroxine and triiodothyronine, and in ordinary sea water. Solutions were changed weekly. The flagellate Dunaliella primolecta Butcher was supplied as food.All the larvae metamorphosed at approximately the same time and rate, two exceptions being that 0.04% thiouracil was a more obviously toxic concentration and the thyroxine and triiodothyronine larvae were rather more precocious in their metamorphosis.It is suggested that metamorphosis is a ‘triggered’ process and is self-sustaining. Thus once it has begun it cannot be delayed or accelerated significantly. All the larvae caught were at a stage when this ‘triggering’ had been released.It is considered possible that the metabolism of the larva and the initiation of metamorphosis is controlled by a secretion of the club-shaped gland.INTRODUCTIONThe position of the amphioxides larva has already been discussed (Wick-stead, 1964). No clear demarcation could be made between the usual type of acraniate larva and amphioxides. It was suggested that the amphioxides was a giant larva, a phenomenon not uncommon in oceanic plankton.IntroductionThe position of the amphioxides larva has already been discussed (Wickstead, 1964).

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