A sub-ectodermic plexus of nerve fibrils, with nodal ganglion cells, has been found in some Ctenophora. Sub-ectodermic muscle cells occur on the body of Hormiphora, of Eucha?is, on its broad aspect in Cestus: in the last longitudinally arranged. Circular fibres surround the tentacle sheaths, and the tactile papillae of Eucharis have a longitudinal coat.
The central nervous system or sensory area consists of delicate columnar ciliated cells. The bell is produced by the fusion of very long cilia in the larva: so too the four plates which support the otolithic mass. The meridional ciliated furrows or nerves commence at the bases of these plates as so many lines of ciliated cells, each line then dividing into two. The otoliths are formed in cells near the longer sides of the area, extruded and flicked against the mass to which they adhere. The margins of the polar plates consist of flagellate columnar cells, their central areae of flat polygonal cells, each with a cilia-plate.
The ctenophoral plates are transparent, rectangular, with margins somewhat split up, and composed of innumerable agglutinated cilia of relatively enormous length, derived from transverse ridges of cells. Their bases are usually close set, or connected by a few cells, or by a nerve in Lobatae and the young Cestus. The Mertensid Charistephane has but two plates in each row: in others they are numerous. Each plate is bent adorally near its base, but the bent portion is concave aborally, the direction in which it moves. Movement of one or of all the plates supporting the otolithic mass, is propagated adorally down the corresponding nerve. The animal can reverse the direction: it may also retain its position unchanged while the plates are moving.
The tentacles are solid, derived entirely from the ectoderm of their basis2. Their axis is gelatinous, divided by a septum in which are imbedded fine nerve-fibres, into a right and left half. Each half contains a bundle of muscle fibres.
1Von Lendenfeld believes that these bristles are defensive and not tactile, and that the gland cells scattered among them, e. g. on the papillae of Eucharis, are poison glands: Z. W. Z. xli. p. 679.
2So say Chun and R. Hertwig, but see note 2, p. 720.
A single fibre is oval or circular in section, homogeneous, nucleated when developing. The superficial cells of the axis are ordinary ectoderm cells in Euplocamis stationis, in others tactile cells, few in number, and adhesive cells which are replaced in Euchlora (Oiuenid) rubra by cnidoblasts on the main axis, but not its branches. An adhesive cell has a convex free surface studded with adhesive globules (p. 331), and contains a muscle filament in connection with the fibres of the axis. The outer part of the filament is spirally coiled to admit of extension when the cell is forcibly elongated by the struggles of the prey. The branches of a tentacle, and the lateral tentacles of Lobatae and Cestus, have the same structure as the main tentacle.
The mesoglaea is firm in Cestidae and Beroidae, soft in others. It imbeds (I) amoeboid nucleated 'connective tissue' cells; (2) fine irregularly branched nervous filaments connected one to another, and to the muscle fibres1; (3) muscle fibres, composed of a sarcolemma, a clear cortex, a granular axial medulla with nuclei; few in number in the Cestidae alone; disposed radially between the stomach, funnel, ctenophoral vessels, and the surface of the body; circularly round the stomach and scantily round the body near its surface; meridionally near the surface, between the ctenophoral rows. The radial fibres are branched at each end, the others pointed. Circular fibres surround the mouth in Cydippidae and Beroidae, and in all Cteno-phora the sensory area. The latter can be deeply retracted into the mesoglaea: so too the ciliated furrows or nerves, and to a greater or less degree the rows of ctenophoral plates, e. g. in Beroidae.
The stomach is lined by ectoderm, similar to that of the body-surface; 'glance' cells are sometimes wanting. Two stomachal ridges, covered chiefly by glandular or granular cells, vis-a-vis to one another, are present except in Beroidae. A layer of longitudinal sub-epithelial muscles, and a nervous plexus external to it, are found in the stomach of that family, the musculature in other families as well.
The endoderm of the funnel and the vessels is unilaminar: its cells flat, polygonal, ciliated, except in the ampullae of the funnel vessels, at the sides of the paragastric vessels, and the outer aspects of the ctenophoral, where they are columnar, vacuolate, non-ciliated (?), and disposed in a single or double ridge. The two forms of cells pass one into the other. 'Ciliated rosettes,' or minute depressions into the mesoglaea, lined by small ciliated cells, occur in the flat epithelium of the funnel and vessels. Longitudinal and circular muscles are said to surround the vessels.
The genital organs are invaginations from the ectoderm (Hertwig). In Beroidae they form continuous bands of cells; in Callianira, Hormiphora, Euplokamis close-set masses of cells connected to the ectoderm by cellular cords. The cell-masses grow into the endoderm. The testes have a sinus, except in Beroe, with an outer wall of flattish cells, and an inner of nucleated protoplasm, from which the spermatozoa are derived. The ovaries are solid, the cells of the outer wall vacuolate, of the inner transformed into ova.
The Ctenophora are classified by Chun as follows: - I. Tentaculata. Tentacles present.
1. Cydippidae (= Saccatae): globular or cylindrical forms. Two long tentacles, simple (Euchlora) or beset with lateral branches. All the peripheral vessels end blindly.
1They are possibly connected with the sub-ectodermic nervous plexus. The fine filaments which underlie the ciliary furrows, Chun's nerves, are probably nervous. They extend below the sensory area, where the different bundles anastomose. They are perhaps connected to the ciliated ectoderm cells.
(i.) Mertensidae, body compressed in the stomachal plane: sub-tentacular ctenophoral rows longer than the subventral; Euchlora, Charistephane, etc.
(ii.) Callianiridae, body similarly compressed: aboral pole with wing-like processes; Callianira (=Eschscholtzia).
(iii.) Pleurobrachiadae, body round in aquatorial section: ctenophoral rows similar; Hormiphora (= Cydippe in part), Pleurobrachia, La?npetia, Euplokamis.
2 and 3. Body compressed in the tentacular plane. Lateral tentacles contained in a furrow of the oral margin. Principal tentacle may be present as in Eucharis, or else wanting. Peripheral vessels communicate. Subventral ctenophoral rows longer than the subtentacular. Larval form a Mertensia-like Cydippid.
2. Lobatae, two lobes in the stomachal plane; Bolina, Deiopea, Eucharis, etc.
3. Cestidae, body band-like; Cestus, Vexillum. II. Nuda. No tentacles.
4. Beroidae, the vessels have lateral branches, and communicate inter se, as well as by their branches; Bero'e, Nets, Idya, etc.
'Ctenophorae' Chun, Fauna und Flora des Golfes von Neapel, i. 1880; Bau der Ctenophoren, R. Hertwig. J. Z. xiv. 1880, or 'Studien zur Blattertheorie,' iii. Jena, 1880: cf. L. Agassiz, Contribution to Nat. Hist. United States, iii. 1860, P. 155Nets cordigera, von Lendenfeld, Z. W. Z. xli. 1885; Metamorphosis of Bolina Chuni, Id. Proc. Lin. Soc. of New South Wales, ix. 1885, p. 929.
Movements of ctenophoral plates, Krukenberg, Vergleich. Physiol. Studien i. (3), 1880: cf. Id. ibid, on Eimer.
Summary of researches on Development, Allman, J. L. S. xvi. 1883; Gastrula and formation of Mesoderm, Metschnikoff, Z. W. Z. xlii. 1885.