As might be expected, degeneration in larval structure is correlated with a concealed habit of life.
Further, a definite cuticular membrane is frequently formed and shed at this stage, which corresponds to the nauplius-stage of larval development.
They are absent in the earliest and most primitive B larval forms (nauplius), and appear only late in the course of development, after many of the trunk-limbs are fully formed.
This must not be taken to mean, however, that the medusa is derived from a sessile polyp; it must be regarded as a direct modification of the more ancient free actinula form, without primitively any intervening polyp-stage, such as has been introduced secondarily into the development of the Leptolinae and represents 'a revival, so to speak, of an ancestral form or larval stage, which has taken on a special role in the economy of the species.
In other insects the imaginal disks are less completely disconnected from the superficies of the larval hypodermis, and may indeed be merely patches thereof.
It will be seen elsewhere, however, that whatever view may be held as to the origin of metagenesis in Hydromedusae, in the case of Scyphomedusae (q.v.) no other view is possible than that the alternation of generations is the direct result of larval proliferation.
A third form, Microfilaria diurna, is found in the larval stage in blood, but only in the daytime.
Thus a mass or chain of embryos is produced, lying in a common cyst, and developing as their larval host develops.
An admirable discrimination of the larval and adult characters of the genus Sergestes has been given by H.
Either before or just after turning, the mantle develops a larval shell termed the protegulum, and when this is completed the larva is termed the Phylembryo.