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A remarkable exception is, however, presented in this respect by Cryptoniscus and Entoniscus, remarkable as a confirmation of Darwin's proposition that "parts developed in an unusual manner are very variable," for in the peculiarly-formed pair of feet there exists the greatest possible difference between the three species hitherto observed.
Embryo of Cryptoniscus planarioides, magnified 90 diam. The last pair of feet on the middle-body of the larva, consequently the penultimate pair in the adult animal, is almost always similar in structure to the preceding pair.
But Spence Bate, a careful observer, states that he has seen females of the form of Praniza laden with eggs far advanced in their development. Entoniscus Cancrorum, female, magnified 3 times. Cryptoniscus planarioides, female, magnified 3 times. In this order we meet for the first time with an extensive retrograde metamorphosis as a consequence of a parasitic mode of life.
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