the mode of life of the Land-Crabs is in favour of Thompson. "Once in
the year," says Troschel's 'Handbuch der Zoologie,' "they migrate in
great crowds to the sea in order to deposit their eggs, and afterwards
return much exhausted towards their dwelling places, which are reached
only by a few." For what purpose would be these destructive migrations
in species whose young quit the egg and the mother as terrestrial
animals?) All the Anomura seem likewise to commence their lives as
Zoeae: witness the Porcellanae, the Tatuira (Hippa emerita) and the
Hermit Crabs. Among the Macrura we are acquainted with the same earliest
form principally in several Shrimps and Prawns, such as Crangon (Du
Cane), Caridina (Joly), Hippolyte, Palaemon, Alpheus, etc. Lastly, it is
not improbable, that the youngest brood of the Mantis-Shrimps (Squilla)
is also in the same case.

The most important peculiarities which distinguish this Zoea-brood from
the adult animal, are as follows:--

The middle-body with its appendages, those five pairs of feet to which
these animals owe their name of Decapoda, is either entirely wanting, or
scarcely indicated; the abdomen and tail are destitute of appendages,
and the latter consists of a single piece. The mandibles, as in the
Insecta, have no palpi. The maxillipedes, of which the third pair is
often still wanting, are not yet brought into the service of the mouth,
but appear in the form of biramose natatory feet. Branchiae are wanting,
or where their first rudiments may be detected as small verruciform
prominences, these are dense cell-masses, through which the blood does
not yet flow, and which therefore have nothing to do with respiration.
An interchange of the gases of the water and blood may occur all over
the thin-skinned surface of the body; but the lateral parts of the
carapace may unhesitatingly be indicated as the chief seat of