Arising from the under surface of the disk there are numerous membranous lamellae disposed in a radiating manner around the gastric cavity. These lamellae seem to be suspended from beneath each radiating canal; consequently they are seventy-four in number; and being much folded upon themselves, each has the appearance of being formed of a double membrane. In their interior they exhibit numerous striae of a violet hue, which the microscope shows to constitute the generative apparatus. In some individuals these folded membranes enclose the ova, and in others contain a fluid filled with spermatozoa; so that they evidently represent the ovaria and testes of these Acalephs.

* Ann. des Sci. Nat. for 1841.

Young Medusa (after Sars.)

Fig. 56. Young Medusa (after Sars.) a, the mouth, surrounded with the as yet undeveloped buccal arms; bb bb, ovaria, or testes; c, radiating nutritive canals; d, marginal circle of nutrient vessels; e, oculiform organs; fff, anal apertures situated on the margin of the disk.

1, 2. Cuvieria carisochroma.

Fig. 57. 1, 2. Cuvieria carisochroma.

(308). Ciliograda

In the Ciliograde Acalephae (Ctenophora), the organs of motion consist of narrow bands of vibratile cilia, variously disposed upon the surface of the animal.

(309). In the globular forms of the Beroes (fig. 58), these cilia are arranged in eight longitudinal rows, and appear to be attached to subjacent arches of a firmer consistence than the rest of the body. They are generally quite naked, but in Pandora are lodged between folds of the skin, which afterwards close over and completely conceal them; their motion is extremely rapid, and sometimes only recognizable by the currents they produce, or by the iridescent hues that play along the arches.

1. Cydippe pileus: a, tentacula unfolded. 2. Supposed nervous system. 3, 4. Isolated cilia.

Fig. 58. 1. Cydippe pileus: a, tentacula unfolded. 2. Supposed nervous system. 3, 4. Isolated cilia.

(310). The arrangement of the locomotive apparatus appended to the eight longitudinal costal bands is extremely beautiful. The series of vibratile fringes is attached to a row of minute transverse ridges, disposed almost like the steps of a ladder, and, moreover, in their essential structure they differ very materially from vibratile cilia of the ordinary character. In shape they are not filiform, but resemble membranous laminae deeply fringed around their free margin, having the shape of so many little semi-oval palletts. The movements of these flabelliform appendages are very rapid, and are seldom interrupted while the animal is in vigorous health; the slightest contact, however, is sufficient to stop them. The different laminae, moreover, belonging to the same row are quite independent of each other; neither does interference with one produce the slightest effect upon the action of the rest. The animal, nevertheless, seems to possess the power of arresting or controlling their motions at pleasure.

It is likewise remarkable that the vibratory movement is kept up for a very long time in fragments separated from the rest of the body, without at all changing its character; but it may be observed that, in portions thus detached, the sensibility appears to be destroyed before the contractile power, inasmuch as, after a certain time, the vibration is kept up unintermittingly in spite of such contact as would previously have caused a suspension of vibratory action.

(311). The cilia, which are placed on the longitudinal ridges, are linear-lanceolate in form, flat, and not hollow. They are not webbed together, and have no communication with the vessels that run beneath the ciliary ridges. Each row of cilia is mounted on a transverse base, of a more solid texture and less transparent than the rest of the body. The substance of this base consists of globules irregularly imbedded in a homogeneous substance*. When one of the cilia of a Cydippe is cut off, it has of itself no power of motion; but if the smallest portion of the substance of its base remains attached, it moves with great vivacity. Hence it is concluded that the ciliary motion is effected by undulatory movements of this peculiar tissue.

(312). In the Beroeform Acalephae, it would seem that the vital principle was equally diffused throughout every part of their fragile substance, which the slightest violence is sufficient to break up into pieces; indeed it is not uncommon to find the surface of the sea covered with fragments of their bodies, on which the locomotive cilia may still be seen in rapid action, producing, by their decomposition of the light, a splendid iridescent appearance.

(313). The capacious cavity that occupies almost the entire length of the body of the Beroe, and communicates freely with the exterior through the inferior orifice, is perfectly smooth internally, and constitutes a kind of wide pharyngeal sac, at the bottom of which is situated a transverse aperture guarded by two thickened lips, the texture of which is firmer than that of the rest of the body. These lips only come in contact with each other near the centre of their free margins, and consequently leave on each side a gaping orifice. The cavity that they thus partially close is very small, and evidently corresponds with the central stomach of the discophorous Medusae, and in like manner constitutes a central reservoir, from whence the vascular system is derived.

(314). The digestive receptacle is filled with a fluid that is continually in movement, and which may be seen to pass into two lateral tubes that soon each divide into four branches and, arriving at the surface of the body, terminate in eight longitudinal canals that convey the contained fluid to the cilia, which latter organs, as they are in constant vibration, appear to perform the functions of a respiratory apparatus. From the lateral parietes of each of the eight longitudinal costal canals there arise an infinite number of small vessels or transverse sinuses; these, after intercommunicating with each other, are lost in the surrounding parenchyma*. Arrived at the margin of the wide opening situated at the inferior extremity of the body, the eight longitudinal trunks terminate in a transverse annular canal that communicates in its turn with two vertical trunks much more deeply seated than the preceding vessels: these lateral vessels, mounting upwards, terminate in the stomachal cavity.