Water Slaters with the "green gland" or "shell-gland" of other
crustacea, assuming that the green gland has no efferent duct and
appealing to the fact that the two organs occur "in the same place."
This interpretation is by no means a happy one. In the first place we
may easily ascertain in Leucifer, as was also found to be the case by
Claus, that the "green gland" really opens at the end of the process
described by Milne-Edwards as a "tubercule auditif" and by Spence Bate
as an "olfactory denticle." And, secondly, the position is about as
different as it can well be. In the one case a paired gland, opening at
the base of the posterior antennae, and therefore on the lower surface
of the SECOND segment; in the other an unpaired structure rising in the
median line of the back BEHIND THE SEVENTH SEGMENT, ("behind the
boundary line of the first thoracic segment," Leydig).) That the last
pair of feet of the thorax is wanting in the young of the Wood-lice
(Porcellionides, M.-Edw.) and Fish-lice (Cymothoadiens, M.-Edw.) has
already been noticed by Milne-Edwards. This applies also to the
Box-Slaters (Idothea), to the viviparous Globe-Slaters (Sphaeroma) and
Shield-Slaters (Cassidina), to the Bopyridae (Bopyrus, Entoniscus,
Cryptoniscus, n.g.), and to the Cheliferous Slaters (Tanais), and
therefore probably to the great majority of the Isopoda. All the other
limbs are usually well developed in the young Isopoda. In Tanais alone,
all the abdominal feet are wanting (but not those of the tail); they are
developed simultaneously with the last pair of feet of the thorax.

(FIGURE 39. Embryo of Cryptoniscus planarioides, magnified 90 diam.

FIGURE 40. Last foot of the middle-body of the larva of Entoniscus
Porcellanae, magnified 180 diam.)

The last pair of feet on the middle-body of the larva, consequently the