penultimate pair in the adult animal, is almost always similar in
structure to the preceding pair. A remarkable exception is, however,
presented in this respect by Cryptoniscus and Entoniscus,--remarkable as
a confirmation of Darwin's proposition that "parts developed in an
unusual manner are very variable," for in the peculiarly-formed pair of
feet there exists the greatest possible difference between the three
species hitherto observed. In Cryptoniscus (Figure 39) this last foot is
thin and rod-like; in Entoniscus Cancrorum remarkably long and furnished
with a strongly thickened hand and a peculiarly constructed chela; in
Entoniscus Porcellanae very short, imperfectly jointed, and with a large
ovate terminal joint (Figure 40).

Some Isopods undergo a considerable change immediately before the
attainment of sexual maturity. This is the case with the males of Tanais
which have already been noticed, and, according to Hesse, with the
Pranizae, in which both sexes are said to pass into the form known as
Anceus. But Spence Bate, a careful observer, states that he has seen
females of the form of Praniza laden with eggs far advanced in their
development.

(FIGURE 41. Entoniscus Cancrorum, female, magnified 3 times.

FIGURE 42. Cryptoniscus planarioides, female, magnified 3 times.

FIGURE 43. Embryo of a Corophium, magnified 90 diam.)

In this order we meet for the first time with an extensive retrograde
metamorphosis as a consequence of a parasitic mode of life. Even in some
Fish-lice (Cymothoa) the young are lively swimmers, and the adults
stiff, stupid, heavy fellows, whose short clinging feet are capable of