176 PHYLUM C@LENTERA. 
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, Coelentera, 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 gastru/a, 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 
(Ascons), the simplest Ccelentera (Hydra), and even by the simplest 
“worms” (Turbellarians). 
If we begin in our survey with such a gastrula-like ancestor, the 
probabilities are certainly in favour of the supposition that it was a free- 
swimming organism. A gradual perfecting of the locomotor character- 
istics might yield the two medusoid types of which we have already 
spoken. But we know that the common jelly-fish Aurelia has a 
prolonged larval stage which is sedentary, vegetative, and prone to bud. 
If we suppose with W. K. Brooks that many forms, less constitutionally 
active than others, relapsed into this sedentary state, with postponed 
sexuality, and with a preponderant tendency to bud, we can understand 
how polyps arose, a these of two types, one nearer the jelly-fish and 
Lucernarians and leading on to sea-anemones and corals, the other 
nearer the swimming-bell type and leading on to a terminus in Hydra. 
It is certainly suggestive that we have jelly-fish wholly free (Pelagia), 
jelly-fish with a sedentary larval life (Azre/éa), jelly-fish predominantly 
passive (Zucernaria), and related polyps (Sea-anemones, etc.), which 
only occasionally rise into free activity; while in the other series we 
have medusoid types always free (Trachymedusze), others which are 
liberated from (Campanularian and Tubularian) sedentary hydroids, 
other (Sertularian and Plumularian) zoophytes whose buds though often 
medusoid-like are not set free, and finally Hydra, which, though it 
may creep on its side, or walk on its head, is predominantly a sedentary 
animal, without any youthful free-swimming stage, 
Ccology.—The Ccelentera are almost all marine. In 
resh water we find the common Aydra, the minute A/icro- 
hydra without tentacles, the strange olypodium, which in 
