Adult attached, enclosed in an integumentary sac, within which a many-valved shell is typically developed. Antenna modified for adhesion. Abdomen rudimentary. Limbs usually present, in the form of multiarticulate cirri. Sexes generally united. Young locomotive.
This sub-class includes, amongst others, the common Acorn-shells and the Barnacles or Goose-mussels. All the Cirripedia are distinguished by the fact, that in the adult condition they are permanently fixed to some solid object by the anterior extremity of the greatly metamorphosed head; the first three cephalic segments being much developed, and enclosing the rest of the body. The larva is free and locomotive, and the subsequent attachment, and conversion into the fixed adult, is effected by means of a peculiar secretion, or cement, which is discharged through the antennae of the larva, and is produced by a special cement-gland, which is really a portion of the ovary. In the Cirripedia, therefore, the head of the adult is permanently fixed to some solid object, and the visceral cavity is protected by an articulated calcareous shell, or by a coriaceous envelope. The posterior extremity of the animal is free, and can be protruded at will through the orifice of the shell. This extremity consists of the rudimentary abdomen, and of six pairs of forked, cirrated limbs, which are attached to the thorax, and serve to provide the animal with food. The two more important types of the Cirripedia are the Acorn-shells (Balanidae) and the Barnacles (Lepadidae). In the former the animal is sessile, the larval antennae, through which the cement exudes, being embedded in the centre of the membranous or calcareous "basis" of the shell. In the latter the animal is stalked, and consists of a "peduncle" and a "capitulum." The peduncle consists of the anterior extremity of the body, with the larval antennae, usually cemented to some foreign body. The capitulum is supported upon the peduncle, and consists of a case composed of several calcareous plates, united by a membrane, enclosing the remainder of the animal.
Fig. 142. - Morphology of Cirripedia. A, Lepas pectinata, one of the Barnacles, one side of the shell being removed, enlarged four times : c Peduncle ; d Cement-duct; o Ovary; s Ovisac; v Vas deferens; p Penis. B, Paecilasma fissa, enlarged five times ; c Peduncle. C, Balanus balanoides, viewed from above, of the natural size. D, Balanus tintinnabulum, with the shell on one side removed to show the animal: a One of the valves ("scutum") of the operculum; b Another valve ("tergum") of the operculum. (After Darwin and Pagenstecher.)
As regards the development of the Cirripedia, the larva has the form of a "Nauplius" (fig. 143, A), with an unsegmented, pyriform body, a median eye, and a dorsal carapace. During its life as a Nauplius, the young moults several times (seven times in Lepas fascicularis, which is here taken as exemplifying the development of the Cirripedia in general); and these various castings of its integuments are accompanied with material changes of form.
Fig. 143. - Development of Lepas fascicularis. A, Early stage of the Nauplius, showing the three pairs of appendages, of which the hinder two pairs are bifurcate: o Eye-spot; b Labrum; g Gullet; h h Lateral horns. B, the free-swimming Cypris-stage or "pupa," after the sixth moult, the antennae and feet retracted within the shell: an Antenna, with its suctorial disc, traversed by the duct of the cement-gland (cg); sg Shell-gland; o Eye ; ad Adductor-muscle; f Feet; c Caudal process. Both figures are greatly enlarged. (After Von Willemoes-Suhm.)
When fully grown, the Nauplius has an oval or pyriform body, enclosed in a carapace, provided with long caudal and dorsal spines. There are three pairs of limbs, of which the first pair (representing the antennas) are undivided, while the two hinder pairs (fig. 143) are bifid, and all carry natatory bristles. There is a very large labrum (fig. 143, b) placed in front of the mouth, and there is a well-developed alimentary tube, which terminates by a distinct anus at the root of the caudal spine. There is at first merely a simple central eye; but in the adult Nauplius, two compound lateral eyes are developed in addition. Ultimately, the Nauplius passes into its second condition or "Cypris-stage" (fig. 143, B), when it is often spoken of as a "pupa." It is now enclosed in an oval, bivalved, mussel-shaped shell, with an opening along the ventral margin. The second and third pairs of the appendages of the Nauplius have now disappeared, and the first pair of appendages constitute strong four-jointed antennae, the last segment of which is disc-shaped, and is pierced centrally by a pore, which is the opening of the excretory duct of the "cement-glands," these organs being situated at the bases of the antennae. The thorax has developed upon its sides six pairs of forked natatory limbs ; and the abdomen is rudimentary, three-jointed, with terminal forked swimming-appendages. The pupa does not feed, but is nourished by means of an extensive accumulation of fatty matter, which had been stored up by the Nauplius in the cephalic and dorsal regions of the body ; while the great labrum of the latter is now very much reduced in size.