Page 290 - Introduction to Paleobiology and The Fossil Record

THE BASAL METAZOANS: SPONGES AND CORALS 277

tentacles
mouth and gullet
tentacles

polyp wall
mesentery
mesentery
basal
infolding septum or
over scleroseptum
septum
basal plate
(a)
tentacles
tentacles

abandoned
corallite

(b)

(c)

(d)

Figure 11.18 Main cnidarian body plans: (a) generalized scleractinian polyp, (b) generalized part

of scleractinian coral colony, (c) living anemone, and (d) living jellyfish. (From various sources.)

include freshwater and colonial forms together phylum. Hydrozoans reproduce either sexu-

with the fire corals and most kinds of “jelly- ally or by asexual budding; the polyp stage is

fish”. There are over 3000 living species asexual and the medusoid normally sexual.
inhabiting water depths up to 8000 m, mainly The scyphozoans are mainly free-swimming

in marine environments. Supposed hydrozo- medusae or jellyfish often inhabiting open-
ans have been recorded from the Late Precam- ocean environments. Some elements of the
brian Ediacara fauna (see p. 242), where Ediacara fauna may be scyphozoans, for
genera such as Eoporpita and Ovatoscutum example Conomedusites and Corumbella;
may be the oldest sessile members of the however many of the best-preserved fossil