with the last pair of feet of the thorax. Fig. 38. Embryo of a Philoscia in the egg, magnified. Fig. 39. Embryo of Cryptoniscus planarioides, magnified. Fig. 40. Last foot of the middle-body of the larva of Entoniscus Porcellanæ, magnified. Fig. 38. Embryo of a _Philoscia_ in the egg, magnified. Fig. 39. Embryo of _Cryptoniscus planarioides,_ magnified. Fig. 40. Last foot of the middle-body of the larva of _Entoniscus Porcellanæ,_ magnified. The last pair of feet on the middle-body of the larva, consequently the 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_ (Fig. 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 Porcellanæ_ very short, imperfectly jointed, and with a large ovate terminal joint (Fig. 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 _Pranizæ,_ 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. Fig. 41. Entoniscus Cancrorum, female, magnified. Fig. 42. Cryptoniscus planarioides, female, magnified. Fig. 43. Embryo of a Corophium, magnified. Fig. 41. _Entoniscus Cancrorum,_ female, magnified. Fig. 42. _Cryptoniscus planarioides,_ female, magnified. Fig. 43. Embryo of a _Corophium,_ magnified.