158 CCELENTERA. 



ably 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, witness, for example, the very 

 abundant Graptolite fossils, which, though probably Ccelentera, do not 

 fit well into any of our modern classes. 



Pedigree. — 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 Hseckel and other 

 naturalists were led to fix their attention especially on the 

 two layered sac or gastrtila, because this form keeps con- 

 tinually 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 

 (Ascones), 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 characteristics 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 

 polypes 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 swimirting bell type and leading 

 on to a terminus in Hydra. It is certainly suggestive that 



