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Updated: May 10, 2025
Nauplius-brood. Ord. 1. Ceratometopa. Nauplius with frontal horns. Ord. 2. LEIOMETOPA. Nauplius without frontal horns. Sub-class II. HEMISCHISTA. Segmentation not complete. A. Nototropa. Embryo bent upwards. Ord. 3. Protura. The tail is first formed. Ord. 3. Saccomorpha. A maggot-like larva-skin is first formed. B. Gasterotropa. Embryo bent ventrally. Ord. 5. Zoeogona.
An inner layer of the larva-skin separates from the outer, and, by changes in the form of the muscles, is drawn into different positions, such as is assumed by the pupa, which thus lies concealed beneath the larva-skin.
We might urge that, according to this proposition, provisional organs as the first produced must exceed the later-formed permanent organs in importance. But let us stick to the Crustacea. In Polyphemus Leydig finds the first traces of the intestinal tube even during segmentation. In Mysis a provisional tail is first formed, and in Ligia a maggot-like larva-skin.
The body having outgrown the larva skin, by a strong muscular effort a rent opens along the back of the thorax, and the insect having fastened its claws into some object at the bottom of the pool, the pupa gradually works its way out of the larva-skin. It is now considerably larger than before. Immediately after this tedious operation, its body is soft, but the crust soon hardens.
In these also the brood bursts out in the Nauplius-form, and speedily strips off its earliest larva-skin which is distinguished by no peculiarities worth noticing. A pair of similar filaments spring, in the larvae of the Cirripedia and Rhizocephala, directly from the brain. Nauplius of Sacculina purpurea, shortly before the second moult, magnified 180 diam.
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