146 CCELENTERATA OR STINGING-ANIMALS. 



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 Coelenterates, do not fit well into any of our 

 modern classes. 



Pedigree. — As to the pedigree of the stinging-animals, 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. One fact is practically 

 certain, that the ancestral many-celled animals — ancestral 

 to Sponges, Coelenterates, and all the rest — were small two- 

 layered tubular or oval forms. The many-celled animals 

 must have begun as clumps of cells ; the question is, what 

 sort of clumps— 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 

 (Ascones), the simplest Coelenterates {Hydra), and even by 

 the simplest " worms " (Turbellarians). In fact, we take 

 sponges first, stinging-animals second, and " worms " third, 

 because their simplest forms are least removed from this 

 gastrula type, — the hypothetical ancestral Gastrma. 



Well, if we begin in our survey from such a gastrula-like 

 ancestor, the probabilities are certainly in favour of the sup- 

 position 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 jellyfish 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 constitu- 

 tionally 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 jellyfish and Lucernarians and 

 leading on to sea-anemones and corals, the other nearer the 



