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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.
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.
Among the males of these various Bopyridae, that of Entoniscus Porcellanae occupies the lowest place; it is confined all its life to six pairs of feet, which are reduced to shapeless rounded lumps. The Amphipoda are distinguishable from the Isopoda at an early period in the egg by the different position of the embryo, the hinder extremity of which is bent downwards.
Considering this uniformity presented by the heart in the entire order of the Amphipoda, it cannot but seem very remarkable, that in the very next order of the Isopoda, we find it to be one of the most changeable organs. Heart of a young Cassidina. Heart of a young Anilocra. Abdomen of the male of Entoniscus Cancrorum. h. Heart. l. In all other Isopoda the heart is removed towards the abdomen.
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