A sense-cell is long, slender, provided with a cilium or stiff hair at the external end, and prolonged basally into 2-3 fine filaments which are connected to nerve fibres or processes of ganglion cells. A supporting cell is usually short and stoutish, its basal end prolonged into filaments which probably enter the mesoglaea. Such supporting cells occur wherever sense cells are present in numbers. In the eye-specks their outer ends are pigmented. The other terms require no explanation.
As to the endoderm, skeletal cells occur in the tentacles of many Hydrozoa, cf. p. 329 on Fig. 7, PI. xiv, under C; skeletogenous cells are met with in some Anthozoa Alcyonaria (?); sense and ganglion cells have been detected in the prostomium of the Hydroid Eucopella and on the mesenteries of Hexactinians.
For cnidoblasts, see p. 330, on Fig. 8, C, and for their contained nematocysts p. 331, on Fig. 9, both PI. xiv.
When the ectoderm is multilaminar, its deeper cells may be undifferentiated, and of irregular outline as in some Hydroids, e. g. Hydra, stem of Endendrium, etc., and they are then termed sub-epidermic or interstitial cells. If they are differentiated three layers are generally distinguishable, an outer of epithelium, gland, and sense cells, etc, a middle of ganglion cells, and an inner of muscle cells, e. g. in Hexactiniae among Anthozoa. A multilaminar endoderm is very rare, but is seen in the stem of a few Hydroids, e. g. Tubularia, or in the hydranth of Myriothela, and perhaps in some Anthozoa.
The archenteron, or gastric cavity, is primitively more or less flask-shaped, but it may be produced into a system of canals, regular in Ctenophora, irregular in many Porifera, or be partially obliterated by the cohesion of its walls, as in the Medusa of Hydrozoa, or broken up by the development of radial ridges (mesenteries and septa) and a stomodaeal invagination in Anthozoa. In colonial forms it is continued through the stems, roots, etc, connecting the zooids; and inasmuch as it is ciliated either partially (most Porifera) or wholly (other Coelenterata), and the products of digestion may therefore be carried through all its parts from the special region where digestion occurs, it is often spoken of analogically as the 'gastrovascular system.' The Ctenophora are remarkable for possessing an ectodermic invagination or 'stomach' in which digestion takes place. The stomodaeum of Anthozoa is simply oesophageal in function.
Intracellular digestion has been observed in a variety of Coelenterata, the epithelial cells possessing in many instances the power of emitting pseudopodia and seizing nutrient particles. The cells may be those of the ectoderm, as in the machopolypes of certain Plumu/aridae, or in certain embryo or larval Hexactinians (Actinia mesembryanthemum: Bunodes sabelloides), or most commonly of the endoderm (Beroe; Sagartia, Aiptasia among Anthozoa; various Hydrozoa). In the Porifera the wandering mesoglaeal cells (in part) take an active share in the process of digestion; and Metschnikoff has compared with this process the destructive power exercised by mesoblast cells in the absorption of parts dead or dying, or of intrusive foreign bodies in Coelomata. See authorities quoted p. 249; cf. Claus, Z. A. iv. 1881. On Porifera, see also von Lendenfeld, Z. W. Z. xxxviii. p. 252, Pole-jaeff's remarks in his 'Report on the Calcarea) Challenger Reports, viii., p. 14, et seqq., and the account of the class, post.
The four classes of Coelenterata seem at the present time to be distinct from one another. The Ctenophora, Anthozoa and Hydrozoa have in common the possession of offensive and defensive cell-structures, adhesive cells or cnidoblasts. The Ctenophora, however, have been recently derived by Haeckel from the Hydrozoa, and from the Anthomedusan family Cladonemidae in particular: see J. Z. xiii. 1879, SB. Jen. Ges. pp. 70-79. Two facts alone, leaving out of sight others, militate strongly if they are not decisive against his view, viz. the absence in all Ctenophora, even in development, of a gastral lamella uniting the endodermal canals, a structure which is present in all Medusae, and the fact that the tentacles are solid ectodermal structures, and do not contain an endodermic axis of any kind as do those of every Hydrozoan, whether hydroid or medusa. The position of the Porifera has been much debated. Histological structure, the presence of sperm, ova, of a blastula and gastrula, prove beyond doubt that a Sponge is not a colony of Protozoa as has been supposed.
Stress has been laid on the absence of tentacles as showing an un-likeness to other Coelenterates; but tentacles are absent in the Ctenophoran Beroidae, in the Hydrozoan Protohydra, Microhydra, Limnocodium and some Medusae. The pores leading to the gastric cavity, or its parts, are a peculiar feature; but aboral pores are found in Ctenophora; pores on the tentacles, peristome, and body-wall and base in some Anthozoa Zoantharia; pores or subumbrellar papillae on the circumferential canal of some Hydrozoan Leptomedusae. These structures are, however, by no means homologous, and the pores of Sponges have a function found in no other Metazoa. Their great development renders possible the irregular and continuous growth of a Poriferan, and is no doubt correlated with a unique phenomenon, the fixation of the animal by the gastrula-mouth, afterwards obliterated, which has been noted in two thoroughly established instances. Add the collared endoderm cells and the absence of cnidoblasts and adhesive cells1, and the sum of peculiarities certainly gives the Porifera an isolated position among Coelenterata, but whether sufficient to erect them into a group of coordinate value is doubtful.
They are retained here in this division of Metazoa on account of the resemblance in their fundamental structure (ecto-, endo-derm and mesoglaea) to what is typical of other Coelenterates in general.
For a remarkable ambulatory colonial Coelenterate, Polypodium ambulans, with non-tentaculate zooids, devoid of mesenteries, but resembling histologically an Actinian, see KorotnerT, Z. A. ix. 1886.