Fig. [51].—Pupa of Lepas pectinata, × 8. A, Antenna; C, carina; M, adductor muscle; S, scutum; T, tergum. (After Gruvel.)
The body of the adult Lepas is retracted into the mantle, and lies free in the mantle-cavity, but is continuous anteriorly with the tissues of the peduncle, into which the mantle does not extend. The thorax, with its six pairs of legs, can be protruded from the mantle-cavity through the slit-like opening which separates the two valves of the mantle along the ventral middle line; and when the animal is feeding, the thoracic legs are so protruded, and by their concerted waving action they drive the food-particles in the water along the channel between them, until the particles reach the oral cone, where they are masticated by the mandibles and two pairs of maxillae, and so passed into the alimentary canal. When the animal is disturbed it rapidly retracts its limbs, the valves of the mantle are closed by means of a strong adductor muscle in the head, and the animal is protected from all external influences. In the acorn-barnacles (Operculata), which live in great numbers attached to rocks and other objects between tide-marks, the body is constructed on a similar plan, save that there is no stalk, and the body is completely enclosed in a hard calcareous box formed from the mantle, which, when the valves are closed, as they always are during low tide, completely protect the animal inside from desiccation or danger of any kind. Besides the cement-glands situated in the peduncle, we can distinguish the generative organs, consisting of a pair of ovaries and testes, the majority of Cirripedes being hermaphrodite. The testes open at the end of an elongated median penis behind the thoracic limbs, while the ovaries, situated in the peduncle, have paired openings into the mantle-cavity on either side of the head. A pair of maxillary glands or kidneys are present, and the alimentary canal is provided with various digestive glands. Special branchial organs are not present in the Pedunculate Cirripedes, but in the Operculate genera two branchiae are formed from the plications of the internal surface of the mantle. There is no contractile heart, and the circulatory system is poorly developed. The Cirripedes are badly furnished with sensory organs; the remains of a simple Nauplius eye may persist, situated on the upper part of the stomach, but the chief sense-organs are the sensory hairs upon the limbs.
Fig. [52].—A, Dwarf male of Scalpellum vulgare, × 27; B, diagram of Stalked Barnacle. a, Peduncle; al, alimentary canal; b, brain; c, carina; e, remains of Nauplius eye; gl, cement-gland; m, mantle-cavity; o, its opening; ov, ovary; p, penis; s, scutum; t, testis; tm, tergum, seen in A as the shaded body above the reference-line of e and to the right of the carina, on the left of the figure.
The recent Cirripedes fall into six clearly defined Sub-orders.
Sub-Order 1. Pedunculata.
In this division, sometimes combined with the Operculata as Thoracica, owing to the extremely reduced state of the abdomen, the body is borne on a distinct stalk, and the bivalve arrangement of the mantle is clearly retained. The mantle is protected externally by a number of calcareous plates, the arrangement of which is typical of the various genera. It appears that in the most primitive and geologically oldest Cirripedes, the probable ancestors of the Pedunculate and Operculate sub-orders, the arrangement of the plates was somewhat irregular, and they were far more numerous than in the modern forms, so that passing from these older types to modern times we witness a reduction in the number and a greater precision in the arrangement of the skeletal parts.
Fig. [53].—A, Turrilepas wrightianus (Silurian), × 1; B, Archaeolepas redtenbacheri (Jurassic), × 1. C, carina; R, rostrum; S, scutum; T, tergum. (After Zittel.)