The pupæ in both orders attach themselves by means of the adherent feet; those of the Cirripedes to rocks, shells, turtles, drift-wood, ships, etc.,—those of the Rhizocephala to the abdomen of Crabs, Porcellanæ, and Hermit Crabs. The carapace of the Cirripedes becomes converted, as is well-known, into a peculiar test, on account of which they were formerly placed among the Mollusca, and the natatory feet grow into long cirri, which whirl nourishment towards the mouth, which is now open. The Rhizocephala remain astomatous; they lose all their limbs completely, and appear as sausage-like, sack-shaped or discoidal excrescences of their host, filled with ova (Figs. 59, 60); from the point of attachment closed tubes, ramified like roots, sink into the interior of the host, twisting round its intestine, or becoming diffused among the sac-like tubes of its liver. The only manifestations of life which persist in these non plus ultras in the series of retrogressively metamorphosed Crustacea, are powerful contractions of the roots, and an alternate expansion and contraction of the body, in consequence of which water flows into the brood-cavity and is again expelled, through a wide orifice.[[8]]
Fig. 59. Young of Peltogaster socialis on the abdomen of a small Hermit Crab; in one of them the fasciculately ramified roots in the liver of the Crab are shown. Animal and roots deep yellow.
Fig. 60. Young Sacculina purpurea with its roots; the animal purple-red, the roots dark grass-green. Magnified.
Figs. 61–63. Eggs of Tetraclita porosa in segmentation, magnified. The larger of the two first-formed spheres of segmentation is always turned towards the pointed end of the egg.
Fig. 64. Egg of Lernæodiscus Porcellanæ, in segmentation, magnified.
Out of several Cirripedes, which are anomalous both in structure and development, Cryptophialus minutus must be mentioned here; Darwin found it in great quantities together in the shell of Concholepas peruviana on the Chonos Islands. The egg, which is at first elliptical, soon, according to Darwin, becomes broader at the anterior extremity, and acquires three club-shaped horns, one at each anterior angle and one behind; no internal parts can as yet be detected. Subsequently the posterior horn disappears, and the adherent feet may be recognised within the anterior ones. From this “egg-like larva”—(Darwin says of it, “I hardly know what to call it”)—the pupa is directly produced. Its carapace is but slightly compressed laterally and hairy, as in Sacculina purpurea; the adherent feet are of considerable size, and the natatory feet are wanting, as, in the adult animal, are the corresponding cirri. As I learn from Mr. Spence Bate, the Nauplius-stage appears to be overleaped and the larvæ to leave the egg in the pupa-form, in the case of a Rhizocephalon (Peltogaster ?) found by Dr. Powell in the Mauritius.
I will conclude this general view with a few words upon the earliest processes in the development of the Crustacea. Until recently it was regarded as a general rule that, by the partial segmentation of the vitellus a germinal disc was formed, and in this, corresponding to the ventral surface of the embryo, a primitive band. We now know that in the Copepoda (Claus), in the Rhizocephala (Fig. 64), and, as I can add, in the Cirripedia (Figs. 61–63) the segmentation is complete, and the embryos are sketched out in their complete form without any preceding primitive band. Probably the latter will always be the case where the young are hatched as true Nauplii (and not merely with a Nauplius-skin, as in Achtheres). The two modes of development may occur in very closely allied animals, as is proved by Achtheres among the Copepoda.[[9]]
[1] If the Phyllopoda may be regarded as the nearest allies of the Trilobites, they would furnish, with Lepidosteus and Polypterus, Lepidosiren and Protopterus, a further example of the preservation in fresh waters of forms long since extinguished in the sea. The occurrence of the Artemiæ in supersaline water would at the same time show that they do not escape destruction by means of the fresh water, but in consequence of the less amount of competition in it.
[2] “The maxilla of the Decapod-larva (Krebslarve) is a sort of Phyllopodal foot” (Claus).
[3] I am still unacquainted with Claus’ latest and larger work, but no doubt the same may be said of it.
[4] The most various opinions prevail as to the position of the Cirripedia. Some ascribe to them a very subordinate position among the Copepoda; as Milne-Edwards (1852). In direct opposition to this notion of his father’s, Alph. Milne-Edwards places them (as Basinotes) opposite to all the other Crustacea (Eleuthéronotes). Darwin regards them as forming a peculiar sub-class equivalent to the Podophthalma, Edriophthalma, etc. This appears to me to be most convenient. I would not combine the Rhizocephala with the Cirripedia, as Liljeborg has done, but place them in opposition as equivalent, like the Amphipoda and Isopoda. The near relationship of the Cirripedia to the Ostracoda is also spoken of, but the similarity of the so-called “Cypris-like larvæ,” or Cirriped-pupæ as Darwin calls them, to Cypris is so purely external, even as regards the shell, that the relationship appears to me to be scarcely greater than that of Peltogaster socialis (Fig. 59) with the family of the sausages.
[5] In connexion with this it may be mentioned that, in the females of Brachyscelus, in which the posterior antennæ are deficient, the conical processes with the canal permeating them are nevertheless retained.