176 PHYLUM CCELENTERA. 



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 

 (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, and 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 (Aurelia), jelly-fish predominantly 

 passive (Lucernaria), and related polyps (Sea-anemones, etc.), which 

 only occasionally rise into free activity ; while in the other series we 

 have medusoid types always free (Trachymedusse), others which are 

 liberated from (Campanularian and Tubularian) sedentary hydroids, 

 other (Sertularian and Plumularian) zoophytes whose buds though 

 medusoid-like are not set free, and finally Hydra, which, though 

 may creep on its side, or walk on its head, is predominantly a sedenta 

 animal, without any youthful free-swimming stage. 



(Ecology. The Ccelentera are almost all marine, 

 resh water we find the common Hydra^ the minute Micro- 

 hydra without tentacles, the strange Polypodium, which in 



