COELENTEEA— JELLY-FISHES. 
45 
•Cambrian Epoch onwards, and all appear to belong to a Gallery X. 
single Sub-Order of Scyphozoa, the Rhizostomajta. (^Itoot- 
uioiiths), so called because the four lips of the mouth are 
drawn out and each fused by its edges into a tulje pierced by 
small openings through which the animal sucks in its food. 
E.Kamples of such fossils from Cambrian, Ordovician, and 
.lurassic rocks are exhibited. ^Vmong them several large Wall-c^e 
ones from the lithographic stone of Soleuhofeu, of Kim- 
meridgian age, resemWe that depicted and explained in our 7 & 8. 
Eiffure 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which the polyps can be withdrawn ; these are the Calypto- 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 tlie Gymnoblastea (naked 
buds). An example of the former is the sea-fir, Strtularia 
((bietina, often cast on our shores. Tlie periderm has a main 
stem, with branches diverging from each side alternately ; 
tlie 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. Xot 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-ceise 
(branch-bearers) or Dendroidea (tree- forms), have numerous 10- 
slender forking branches, connected by transverse processes, 
and bearing little thecae, some for the ordinary polyps, others 
modified ]iossibly for reproductive cells. Some of the genera, 
such as Dendrogmptus and Gallocfrcvplus, seem to have been 
fixed to the sea-floor like a Sertulavia. Dicli/onema, however, 
which forms fan-shaped or funnel-shaped colonies, has been 
