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An exception to this rule is presented by the Hyperinae which usually live upon Acalephae. In these the young and adults often have a remarkably different appearance; but even in these there is no new formation of body-segments and limbs, but only a gradual transformation of these parts.*
It is to be supposed that, not only originally but even still, in the larvae of the first Malacostraca, the new body-segments and pairs of limbs are formed singly, first of all the segments of the fore-body, then those of the abdomen, and finally those of the middle-body, and, moreover, that in each region of the body the anterior segments were formed earlier than the posterior ones, and therefore last of all the hindermost segment of the middle-body.
The body-segments and feet, which are sometimes so numerous, are formed gradually from before backwards, without the indication of any sharply-discriminated regions of the body either by the time of their appearance or by their form.
To-day as I was opening a specimen of Lepas anatifera in order to compare the animal with the description in Darwin's 'Monograph on the Subclass Cirripedia, I found in the shell of this Cirripede, a blood-red Annelide, with a short, flat body, about half an inch long and two lines in breadth, with twenty-five body-segments, and without projecting setigerous tubercles or jointed cirri.
Embryo of a Squilla, magnified 45 diam. a. heart. The corresponding body-segments are already well developed, an unpaired eye is still present, the anterior antennae are already biramose, whilst the flagellum is wanting in the posterior, and the mandibles are destitute of palpi; the four anterior abdominal segments bear biramose natatory feet, without branchiae; the fifth abdominal segment has no appendages, and this is also the case with the tail, which still appears as a simple lamina, fringed on the hinder margin with numerous short teeth.
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