Fam. 4. Anaspidae.—This includes the remaining pedunculate genera, characterised by the fleshy nature of the mantle and peduncle, which are both entirely devoid of calcifications. The species of Alepas live upon Echinoderms and various other animals; Chaetolepas upon Sertularia, and Gymnolepas upon Medusae. Anelasma squalicola is an interesting form, living parasitically upon the Elasmobranch fishes, Selache maxima and Spinax niger in the North Sea. The peduncle is deeply buried in the flesh of the host, so that only a portion of the dark blue capitulum protrudes to the surface. From the whole surface of the peduncle a system of branching processes is given off, which ramify far into the tissues of the fish, and communicate inside the peduncle with the lacunar tissue, which is packed round all the organs of the Cirripede. There can be small doubt that the Anelasma derives its nutriment parasitically through this root-system, since the cirri are mere fleshy lobes unadapted to securing food, and the alimentary canal is always empty. This animal has a suggestive bearing on the Rhizocephala, which, as will be shown, derive their nutriment from a system of roots penetrating the host and growing out from what corresponds morphologically to the peduncle.
Sub-Order 2. Operculata.
Fig. [61].—Balanus tintinnabulum, with the right half of the shell and of the operculum removed, seen from the right side. A, Antennae, the size of which is exaggerated; A.M, adductor muscle; B, basis; C, carina; Cr, cirri or thoracic appendages; D, oviduct; G, ovary; L, lateral compartment; Lb, labrum or upper lip; M, M, depressor muscles of scutum and tergum; M.C, mantle-cavity; O, orifice of excretory organ; O.M, opercular membrane; R, rostrum; S, scutum; St, region of stomach; T, tergum. (After Darwin.)
The “acorn-barnacles” appear later in geological time than the earlier stalked forms. Verruca and Chthamalus are found in the Chalk, and survive down to the present day, but Balanus does not occur until middle Tertiary times. Representatives of the last-named genus are familiar to every one, as the hard sharp objects which cover rocks and piles near high-water mark on every sea-coast. If we examine the hard skeleton of one of these animals, we find that, unlike the Pedunculata, they possess no stalk, the capitulum being fused on to the surface of attachment by a broad basal disc. Typically, there may be considered to be eight skeletal pieces forming the outer ring which invests the soft parts of the animal, an unpaired rostrum and carina, and laterally a pair of rostro-lateral, lateral, and carino-lateral “compartments,” as shown in Figs. [60], 63.
The skeletal ring is roofed over by a pair of terga at the carinal end and a pair of scuta at the rostral end; these four plates make up the operculum by which the animal can shut itself completely up in its shell, or between the valves of which it can protrude its limbs for obtaining food.
Fig. [62].—Diagrammatic section of the growing shell of Balanus porcatus. C, Canals; Ct, cuticle; H, hypodermis (= epidermis); H′, part of shell secreted by the hypodermis; Hl, hypodermal lamina; M, part of shell secreted by the mantle. (After Gruvel.)
The relation of the animal to its shell is shown in Fig. [61]. The shell in the Operculata is not merely secreted as a dead structure on the external surface of the epidermis, but represents a living calciferous tissue interpenetrated by living laminae (Fig. [62], Hl) derived partly from the external hypodermis and partly from the lining of the mantle. The hard parts of the shell usually also contain spaces and canals (C).
