It is, however, to be particularly remarked, that the development of the sexual peculiarities does not stand still on the attainment of sexual maturity.
For example, the younger sexually mature males of Orchestia Tucurauna, n. sp., have slender inferior antennæ, with the joints of the flagellum not fused together, the clasping margin (“palm,” Sp. Bate) of the hand in the second pair of feet is uniformly convex, the last pair of feet is slender and similar to the preceding. Subsequently the antennæ become thickened, two, three, or four of the first joints of the flagellum are fused together, the palm of the hand acquires a deep emargination near its inferior angle, and the intermediate joints of the last pair of feet become swelled into a considerable incrassation. No museum-zoologist would hesitate about fabricating two distinct species, if the oldest and youngest sexually mature males were sent to him without the uniting intermediate forms. In the younger males of Orchestia Tucuratinga, although the microscopic examination of their testes showed that they were already sexually mature, the emargination of the clasping margin of the hand (represented in Fig. 50) and the corresponding process of the finger, are still entirely wanting. The same may be observed in Cerapus and Caprella, and probably in all cases where hereditary sexual differences occur.
Fig. 50. Foot of the second pair (“second pair of gnathopoda”) of the male of Orchestia Tucurauna, magnified.
Fig. 51. Foot of the second pair (“second pair of gnathopoda”) of the female of Orchestia Tucurauna, magnified.
Fig. 52. Male of a Bodotria, magnified. Note the long inferior antennæ, which are closely applied to the body, and of which the apex is visible beneath the caudal appendages.
Next to the extensive sections of the Stalk-eyed and Sessile-eyed Crustacea, but more nearly allied to the former than to the latter, comes the remarkable family of the Diastylidæ or Cumacea. The young, which Kröyer took out of the brood-pouch of the female, and which attained one-fourth of the length of their mother, resembled the adult animals almost in all parts. Whether, as in Mysis and Ligia, a transformation occurs within the brood-pouch, which is constructed in the same way as in Mysis, is not known.[[7]] The caudal portion of the embryo in the Diastylidæ, as I have recently observed, is curved upwards as in the Isopoda, and the last pair of feet of the thorax is wanting.
Equally scanty is our knowledge of the developmental history of the Ostracoda. We know scarcely anything except that their anterior limbs are developed before the posterior one (Zenker). The development of Cypris has recently been observed by Claus:—“The youngest stages are shell-bearing Nauplius-forms.”
[1] Leydig has compared this foliaceous appendage of the 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 antennæ, 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).
[2] nIn the genera Orchestoidea, Orchestia, Allorchestes, Montagua, Batea n.g., Amphilochus, Atylus, Microdeutopus, Leucothoë, Melita, Gammarus (according to Meissner and La Valette), Amphithoë, Cerapus, Cyrtophium, Corophium, Dulichia, Protella and Caprella.ote
[3] Little as a name may actually affect the facts, we ought certainly to confine the name “micropyle” to canals of the egg-membrane, which serve for the entrance of the semen. But the outer egg-membrane passes over the “micropylar apparatus” of the Amphipoda without any perforation, according to Meissner’s and La Valette’s own statements; it appears never to be present before fecundation, attains its greatest development at a subsequent period of the ovular life, and the delicate canals which penetrate it do not even seem to be always present, indeed it seems to belong to the embryo rather than to the egg-membrane. I have never been able to convince myself that the so-called “inner egg-membrane” is really of this nature, and not perhaps the earliest larva skin, not formed until after impregnation, as might be supposed with reference to Ligia, Cassidina and Philoscia.
[4] According to Spence Bate, in Brachyscelus crusculum the fifth abdominal segment is not amalgamated with the sixth (the tail) but with the fourth, which I should be inclined to doubt, considering the close agreement which this species otherwise shows with the two species that I have investigated.