COELENTERA TA. 715 



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, &c., 

 and they are then termed sub-epidermic or interstitial cells. If they are differenti- 

 ated three layers are generally distinguishable, an outer of epithelium, gland, and 

 sense cells, &c., a middle of ganglion cells, and an inner of muscle cells, e. g. in 

 Hcxactiniac 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, &c., 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 epi- 

 thelial 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 Plumularidae, or in certain embryo or larval Hexactinians 

 (Actinia mesembryanthemum : Bunodes sabelloides), or most commonly of the endo- 

 derm (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. 

 1 88 1. On Porifera, see also von Lendenfeld, Z. W. Z. xxxviii. p. 252, Pole- 

 jaeff's remarks in his ' Report on the Calcareaj 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 pos- 

 session 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 



