Fig. [66].—Nearly median longitudinal section (diagrammatic) of Peltogaster. gn, Brain; m, mantle; mc, mantle-cavity; mes, mesentery; op, mantle-opening; ov, ovary; ovd, oviduct; ring, ring of attachment; t, testis; vd, vas deferens.
Fig. [67].—Diagrammatic median longitudinal section through a normal Cirripede, gn, Brain; op, mantle-opening; ovd, oviduct; vd, vas deferens.
The structure of the external bag-like portion is very simple, and varies only in details, chiefly of symmetry, in the different genera. In Peltogaster, which preserves the simplest symmetrical arrangement of the organs, a diagrammatic section through the long axis of the body (Fig. [66]) shows that it consists of a muscular mantle (m) surrounding a visceral mass, and enclosing a mantle-cavity (mc) or brood-pouch, which stretches everywhere between mantle and visceral mass, except along the surface by which the parasite is attached to its host, where a mesentery (mes) is formed. The ring of attachment is situated in the middle of this mesentery; the mantle-cavity, which is completely lined externally and internally with chitin, opens anteriorly by means of a circular aperture (op) guarded by a sphincter muscle. The visceral mass is composed chiefly of the two ovaries (ov), which open on either side of the mesentery by means of a pair of oviducts (ovd); the paired testes (t) are small tubes lying posteriorly in the mesentery, and the nervous ganglion (gn) lies in the mesentery between oviducts and mantle-opening. A comparison with the condition of a normal Cirripede (Fig. [67]) shows us that the mesenterial surface of the parasite by which it is fixed corresponds to the dorsal surface of an ordinary Pedunculate Cirripede, and that the ring of attachment corresponds with the stalk or peduncle of a Lepas. The root-system passes out through the ring of attachment into the body of the host, and ramifies round the organs of the crab; the roots are covered externally with a thin chitinous investment, and consist of an epithelium and an internal mass of branching cells continuous with the lacunar tissue in the visceral mass.
Fig. [68].—Development of Sacculina neglecta. A, Nauplius stage, × about 70; B, Cypris stage, × about 70. A1, A2, 1st and 2nd antennae of Nauplius; Ab, abdomen; Ant, antenna of Cypris; E, undifferentiated cells; F, frontal horn; G, glands of Cypris; H, tendon of Cypris; M, mandible; T, tentacles.
The developmental history of the Rhizocephala is one of the most remarkable that embryology has hitherto revealed. It has been most accurately followed in the case of Sacculina. The young are hatched out in great numbers from the maternal mantle-cavity as small Nauplii (Fig. [68], A) of a typical Cirripede nature, but without any alimentary canal. They swim near the surface of the sea, and become transformed into Cypris larvae of a typical character (Fig. [68], B). The Cypris larva, after a certain period of free existence, seeks out a crab and fixes itself by means of the hooks on its antennae to a hair on any part of the crab’s body. Various races of Sacculina are known which infest about fifty different species of crabs in various seas; the best known are S. carcini parasitic on Carcinus maenas at Plymouth and Roscoff, and S. neglecta on Inachus mauritanicus at Naples. The antenna, by which the Cypris is fixed, penetrates the base of the hair; the appendages are thrown away, and a small mass of undifferentiated cells is passed down the antenna into the body-cavity of the crab. Arrived in the body-cavity it appears that this small mass of cells is carried about in the blood-stream until it reaches the spaces round the intestine in the thorax. Here it becomes applied to the intestine, usually at its upper part, immediately beneath the stomach of the crab (Fig. [69]), and from this point it proceeds to throw out roots in all directions, and as it grows to extend its main bulk, called the central tumour (c.t), towards the lower part of the intestine. As the posterior border of the central tumour grows down towards the hind-gut, the future organs of the adult Sacculina become differentiated in its substance; the mantle-cavity being excavated and surrounding the rudiment of the visceral mass, while as the central tumour grows downwards it leaves behind it an ever extending system of roots. When the central tumour in process of differentiation has reached the unpaired diverticulum of the crab’s intestine, at the junction between thorax and abdomen, all the adult organs are laid down in miniature, and the whole structure is surrounded by an additional sac formed by invagination known as the perivisceral space (Fig. [70]). The young “Sacculina interna” remains in this position for some time, and being applied to the ventral abdominal tissues of the crab just at the point where thorax and abdomen join, or a little below it, it causes the crab’s epithelium to degenerate, so that when the crab moults, a little hole is left in this region of the same size as the body of the Sacculina, owing to the failure of the epithelium to form chitin here; and thus the little parasite is pushed through this hole and comes to the exterior as the adolescent “Sacculina externa.” From this point onwards the crab, being inhibited in its growth through the action of the parasite, never moults again; so that the Sacculina occupies a safe position protruding from the crab’s abdomen, which laps over it and protects it. The remarkable features of this development are, firstly, the difficulty of understanding how the developing embryo is directed in its complicated wanderings so as always to reach the same spot where it is destined to come to the exterior; and, secondly, the loss after the Cypris stage of all the organs and the resumption of an embryonic undifferentiated state from which the adult is newly evolved. A certain parallel to this history is found in that of the Monstrillidae, described on pp. 64–66.
Fig. [69].—The mid-gut of Inachus mauritanicus with a young Sacculina overlying it, × 2. c.t, “Central tumour” of the parasite; d.i, d.s, inferior and superior diverticula of alimentary canal of host; n, “nucleus,” or body-rudiment of Sacculina; r, its roots; x, definitive position of the parasite.