of the embryo." If the earlier manifested peculiarities are to be
estimated more highly than those which afterwards make their appearance,
then in those cases in which the structure of the adult animal requires
one position in the system, and that of the larva another, the latter
and not the former must decide the point. As the Lernaeae and
Cirripedes, on account of their Nauplius-brood, were separated from
their previous connexions and referred to the Crustacea, we shall, for
the same reason, have to separate Peneus from the Prawns and unite it
with the Copepoda and Cirripedia. But the most zealous embryomaniac
would probably shrink from this course.

A "true and natural system" of the Crustacea to be in accordance with
the sequence of the phenomena would have to take into account in the
first place the various modes of segmentation, then the position of the
embryo, next the number of limbs produced within the egg and so forth,
and might be represented somewhat as follows:--

CLASSIS CRUSTACEA.

Sub-class I. HOLOSCHISTA.--Segmentation complete. No primitive band.
Nauplius-brood.

Ord. 1. Ceratometopa.--Nauplius with frontal horns. (Cirripedia,
Rhizocephala.)

Ord. 2. LEIOMETOPA.--Nauplius without frontal horns. (Copepoda, without
Achtheus, etc., Phyllopoda, Peneus.)

Sub-class II. HEMISCHISTA.--Segmentation not complete.