In accordance with this view there would be also some probability in favour of regarding the collar nerve-tube of the Enteropneusta as the equivalent of the cerebral vesicle only of Amphioxus and the Ascidian tadpole, and also of the primary forebrain of vertebrates.
It is absurd to call the larva of a newt or of a Caecilian a tadpole, nor is the free-swimming embryo of a frog as it leaves the egg a tadpole.
The tadpole of the North American bull-frog measures six inches, and that of the Chilian Calyptocephalus gayi seven and a half inches.
A tadpole is the larva of a tailless Batrachian after the loss of the external gills and before the egress of the fore limbs (except in the aberrant Xenopus) and the resorption of the tail.
What characterizes a tadpole is the conjoined globular head and body, so formed that it is practically impossible to discern the limit between the two, sharply set off from the more or less elongate compressed tail which is the organ of propulsion.
The hind limbs appear as buds at the base of the tail, and gradually attain their full development during the tadpole life.
The circular lip is extremely developed in Megalophrys montana, and its funnel-shaped expansion, beset on the inner side with radiating series of horny teeth, acts as a surface-float, when the tadpole rests in a vertical position; the moment the tadpole sinks in the water the funnel collapses, taking on the form of a pair of horns, curling backwards along the side of the head; but, as they touch the surface again, it re-expands into a regular parachute.
Erasmus Darwin, the grandfather of Charles Darwin, set forth ',in' Zoonomia a much more definite theory of the relation of variation to evolution, and the following passage, cited by Clodd, clearly expresses it: "When we revolve in our minds the metamorphoses of animals, as from the tadpole to the frog; secondly, the changes produced by artificial cultivation, as in the breeds of horses, dogs and sheep; thirdly, the changes produced by conditions of climate and season, as in the sheep of warm climates being covered with hair instead of wool, and the hares and partridges of northern climates becoming white in winter; when, further, we observe the changes of structure produced by habit, as shewn especially by men of different occupations; or the changes produced by artificial mutilation and prenatal influences, as in the crossing of species and production of monsters; fourth, when we observe the essential unity of plan in all warmblooded animals - we are led to conclude that they have been alike produced from a single living filament."
The entire revolution which much of his policy underwent in order to effect this object bears too close a resemblance to the sudden and inexplicable changes of front habitual to placemen of the Tadpole 'stamp to be altogether pleasant to contemplate in a politician of pure aims and lofty ambition.
TADPOLE, a term often, but wrongly, applied indiscriminately to all Batrachian larvae.
If in the course of the first twenty-four hours this larva meet with a tadpole it attaches itself at once and undergoes further development.
Sometimes the Polystomum-larva attaches itself to a young tadpole, and in that case grows so rapidly as to become mature in five weeks.
When the time for eclosion has come, the male enters the water with his burden; the larvae, in the full tadpole condition, measuring 14 to 17 millimetres, bite their way through their tough envelope, which is not abandoned by the father until all the young are liberated, and complete in the ordinary way their metamorphosis.
Palaeobatrachus (26), of which a number of species represented by skeletons of the perfect form and of the tadpole have been described from Miocene beds in Germany, Bohemia and France, seems to be referable to the Pelobatidae; this genus has been considered as possibly one of the Aglossa, but the absence of ribs in the larvae speaks against such an association.
In the former case the larva creeps along the tadpole until it reaches the branchial opening into which it darts, fixes its sucker, and then throws off its cilia.