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Who would suspect this strange atom would turn into a Crab! Well, nobody did. It was called a zoea; but you can call it a Crab caterpillar or larva. The maggot is the larva of the fly, and the zoea is the larva of the Crab. With crowds of its brothers and sisters, the zoea kicks about on the surface of the sea. Fishes, and even great whales, swallow these tiny things by the million.
Nevertheless the supposition that the Insecta had for their common ancestor a Zoea which raised itself into a life on land, may be recommended for further examination. Much in what has been adduced above may be erroneous, many an interpretation may have failed, and many a fact may not have been placed in its proper light.
They consist, exactly as described by Leydig in the Daphniae, of an outer and inner lamina, the space between which is traversed by numerous transverse partitions dilated at their ends; the spaces between these partitions are penetrated by a more abundant flow of blood than occurs anywhere else in the body of the Zoea.
The view that the insects were derived from a Zoëa was also sustained by Friedrich Brauer, the distinguished entomologist of Vienna, in a paper read in March, 1869. Following the suggestion of Fritz Müller and Hæckel, he derives the ancestry of insects from the Zoëa of crabs and shrimps.
No trace of a carapace! no trace of the paired eyes! no trace of masticating organs near the mouth which is overarched by a helmet-like hood! Nauplius of a Prawn, magnified 45 diam. Young Zoea of the same Prawn, magnified 45 diam. Older Zoea of the same Prawn, magnified 45 diam.
Where, on the contrary, respiration remained with the anterior part of the body, whether in the primitive fashion of Zoea, as in the Tanaides, or by the development of branchiae on the thorax, as in the Amphipoda, the primitive form of the heart was inherited unchanged, because any variations which might make their appearance were rather injurious than advantageous, and disappeared again immediately.
The youngest larva of the Schizopod genus Euphausia observed by Claus, stands very near the youngest Zoea of our Prawns; but whilst its anterior antennae are already biramose, and it therefore appears to be more advanced, it still wants the middle maxillipedes. In it also Claus found the heart furnished with only a single pair of fissures.
Do not Nauplius-like states in this case also precede the Zoea? The last segment and the last two pairs of feet of the middle-body are wanting. The developmental history of Mysis, the near relationship of which with the Shrimps and Prawns has recently again been generally recognised, has been described in detail by Van Beneden. So far as I have tested them I can only confirm his statements.
This is another example warning us to be cautious in deductions from analogy. Nothing seemed more probable than to refer back the beak-like formation of the forehead in the Oxyrhynchi to the frontal process of the Zoea, and now it appears that the young of the Oxyrhynchi are really quite destitute of any such process.
Any such assumption as this was, however, very hazardous, so long as not a single fact properly relating to the Edriophthalma could be adduced in its support, as the structure of this very coherent group seemed to be almost irreconcilable with many peculiarities of the Zoea.
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