In addition to the remarkable external metamorphosis undergone during the pupa stage, a series of hardly less considerable internal changes take place, such as the atrophy of the muscles of the antennæ, a change in the position of the stomach, etc.
Abdominalia. In the Alcippidæ the larva leaves the egg as a Nauplius, and this stage is eventually followed by a pupa stage closely resembling that of the Thoracica. There are six pairs of thoracic natatory legs (Darwin, No. [519]). Of these only the first and the last three are preserved in the adult, the first being bent forward in connection with the mouth. The body moreover partially preserves its segmentation, and the mantle does not secrete calcareous valves.
Fig. 233. Stages in the development of the Rhizocephala. (From Huxley, after Fritz Müller.)
A. Nauplius of Sacculina purpurea. B. Cypris stage of Lernæodiscus porcellanæ. C. Adult of Peltogaster paguri.
II. III. IV. Two pairs of antennæ and mandibles; cp. carapace; a. anterior end of body; b. generative aperture; c. root-like processes.
The very remarkable genus Cryptophialus, the development of which is described by Darwin (No. [519]) in his classical memoir, is without a free Nauplius stage. The embryo is at first oval but soon acquires two anterior processes, apparently the first pair of antennæ, and a posterior prominence, the abdomen. In a later stage the abdominal prominence disappears, and the antennary processes, within which the true antennæ are now visible, are carried more towards the ventral surface. The larva next passes into the free Cypris stage, during which it creeps about the mantle cavity of its parent. It is enveloped in a bivalve shell, and the antennæ have the normal cirriped structure. There are no other true appendages, but posteriorly three pairs of bristles are attached to a rudimentary abdomen. Paired compound eyes are present. During the succeeding pupa stage the metamorphosis into the adult form takes place, but this has not been followed out in detail.
In Kochlorine, a form discovered by Noll (No. [526]) and closely related to Cryptophialus, the larvæ found within the mantle represent apparently two larval stages, similar to two of the larval stages described by Darwin.
Rhizocephala. The Rhizocephala, as might have been anticipated from their close relationship to Anelasma squalicola amongst the Thoracica, undergo a development differing much less from the type of the Thoracica than that of Cryptophialus and Kochlorine.
Sacculina leaves the egg as a Nauplius ([fig. 233] A), which differs from the ordinary type mainly (1) in the large development of an oval dorsal shield (cp) which projects far beyond the edge of the body, but is provided with the typical sternal horns, etc.; and (2) in the absence of a mouth. The Cypris and pupa stages of Sacculina and other Rhizocephala ([fig. 233] B) are closely similar to those of the Thoracica, but the paired eye is absent. The attachment takes place in the usual way, but the subsequent metamorphosis leads to the loss of the thoracic feet and generally to retrogressive changes.