which chiefly belong to the land and to fresh water,--to the Planariae
of the fresh waters and the Tetrastemma of the sparingly saline Baltic
among the Turbellaria,--to the Pulmonate Gasteropoda, and to the
Branchiferous Gasteropoda of the fresh waters, the young of which
(according to Troschel's 'Handb. der Zoologie') have no ciliated buccal
lobes, although such organs are possessed by the very similar
Periwinkles (Littorina).

All the marine forms of this section appear to be subject to a more or
less considerable metamorphosis. This appears to be only inconsiderable
in the common Lobster, the young of which, according to Van Beneden, are
distinguished from the adult animal, by having their feet furnished,
like those of Mysis, with a swimming branch projecting freely outwards.
From a figure given by Couch the appendages of the abdomen and tail also
appear to be wanting.

Far more profound is the difference of the youngest brood from the
sexually mature animal in by far the greater majority of the
Podophthalma, which quit the egg in the form of Zoea. This young form
occurs, so far as our present observations go, in all the Crabs, with
the sole exception of the single species investigated by Westwood. I say
SPECIES, and not GENUS, for in the same genus, Gecarcinus, Vaughan
Thompson found Zoea-brood,* which is also met with in other terrestrial
Crabs (Ocypoda, Gelasimus, etc.). (* Bell ('Brit. Stalk-eyed Crust.'
page 45) considers himself justified in "eliminating" Thompson's
observation at once, because he could only have examined ovigerous
females preserved in alcohol. But any one who had paid so much attention
as Thompson to the development of these animals, must have been well
able to decide with certainty upon eggs, if not too far from maturity or
badly preserved, whether a Zoea would be produced from them. Moreover,