COELENTERA — JELLY-FISHES. 



45 



Cambrian Epoch onwards, and all appear to belong to a Gallery X. 



single Sub-Order of Scypliozoa, the Rhizostomata (Koot- 



raouths), so called because the four lips of the mouth are 



drawn out and each fused by its edges into a tube pierced by 



small openings through which the animal sucks in its food. 



Examples of such fossils from Cambrian, Ordovician, and 



Jurassic rocks are exhibited. Among them a large one from Wall-case 



the lithographic stone of Solenhofen, of Kimmeridgian age, 



called Bhizostomites, resembles that depicted and explained 



in our Figure 18. Dr. C. D. Walcott has published a full 



account of fossil jelly-fishes in the Monographs of the United 



States Geological Survey (1898). Except for these imprints, 



the Scyphozoa are not known as fossils. 



Class HYDROZOA. 



Among the Hydrozoa many of the colonial forms are 

 protected by a thin horny coat of chitin secreted by the 

 ectoderm and called "periderm." In some this covers the 

 branches of the colony and forms little cups or thecae, into 

 which the polyps can be withdrawn ; these are the Calypto- 

 blastea (covered buds). In others the periderm does not 

 expand into cups ; these are the Gymnoblastea (naked 

 buds). An example of the former is the sea-fir, Sertularia 

 abietina, often cast on our shores. The periderm has a main 

 stem, with branches diverging from each side alternately ; 

 the sides of the stem and of all the branches are clothed 

 with a row of cups or thecae, also in alternate arrangement. 

 In each theca lives a polyp, which stretches out twenty-four 

 tentacles and is connected with its fellows by a cord of flesh 

 that passes inside each branch and down the main stem. It 

 is strange that, though capable of preservation, no traces of 

 the chitinous periderm of any such hydroid should have been 

 found in either Cainozoic or Mesozoic rocks. Not until we 

 pass back to early Palaeozoic times, Cambrian to Devonian, 

 do we find organisms that bear any resemblance to the 

 Calyptoblastea. These, which are called Cladophora Table-case 

 (branch-bearers) or Dendroidea (tree- forms), have numerous 

 slender forking branches, connected by transverse processes, 

 and bearing little thecae, some for the ordinary polyps, others 

 modified possibly for reproductive cells. Some of the genera, 

 such as Dendrograptus and Callograptus, seem to have been 

 fixed to the sea-floor like a Sertularia. Dictyonema, however, 

 which forms fan-shaped or funnel-shaped colonies, has been 



