152 CCELENTERA. 



like combinations of cilia. The stinging cells are almost always 

 replaced by "adhesive cells." The mouth is at one pole, and leads 

 into an ectodeimic gullet. The gastric cavity is usually much branched. 

 The mesenchyme is very well developed, and includes muscular and 

 connective cells. At the aboral pole there is a sensory organ, including 

 an "otolith," which seems of use in steering. Here, also, there are 

 two excretory apertures. Except in Beroe and its near relatives, there 

 are two retractile tentacles. All are hermaphrodite. The development 

 is direct. They are pelagic, very active in habit, carnivorous in diet, 

 and often phosphorescent. According to Lang, they have affinities 

 with Planarian " worms," but this is very uncertain. 



Examples : — 



(a) With tentacles, Cydippe and the ribbon-shaped Venus' Girdle 



(Cestum Veneris). 



(b) Without tentacles, Beroe. 



History of Ccelentera. — Of corals, as we would expect, the rocks 

 preserve a faithful record, and we know, for instance, that in the 

 older (Paleozoic) strata they were represented by many types. We 

 often talk of the imperfection of the geological record, and rightly, for 

 much of the library has been burned, many of the volumes are torn, 

 whole chapters are wanting, and many pages are blurred. But this 

 imperfect record sometimes surprises us, as in the quite distinct remains 

 of ancient jelly-fish, which animals, as we know them now, are appar- 

 ently little more than animated sea water. We should also grasp the 

 conception, with which Lyell first impressed the world, of the uniformity 

 of natural processes throughout the long history of the earth. Thus in 

 connection with Ccelentera we learn that there were great coral reefs in 

 the incalculably distant past, just as there are coral reefs still. So in 

 the Cambrian rocks, which are next to .the oldest, there are on sandy 

 slabs markings exactly like those which are now left for a few hours 

 when a large jelly-fish stranded on the flat beach slowly melts away. 

 On the other hand, some forms of life which lived long ago, seem to 

 have been very different from any that now remain, as is well shown 

 by the abundant Graptolite fossils, which, though probably Ccelentera, 

 do not fit well into any of the modern classes. 



As to the pedigree of the Ccelentera, the facts of individual life 

 history, and the scientific imagination of naturalists, help us to construct 

 a genealogical tree — a hypothetical statement of the case. Thus it 

 seems very likely that the ancestral many-celled animals — ancestral to 

 Sponges, Ccelentera, and all the rest — were small two-layered tubular 

 or oval forms. The many-celled animals must have begun as clusters 

 of cells ; the question is, what sort of clusters — spheres of one layer of 

 cells, or mouthless ovals, or little discs of cells, or two-layered thimble- 

 like sacs? Possibly there were many forms, but Haeckel and other 

 naturalists were led to fix their attention especially on the two-layered 

 sac or gastrula, because this form keeps continually cropping up as an 

 embryonic stage in the life history of animals, whether sponge or coral, 

 earthworm or starfish, mollusc or even vertebrate, and also because this 

 is virtually the form which is exhibited by the simplest sponges 



