Page 288 - Introduction to Paleobiology and The Fossil Record

THE BASAL METAZOANS: SPONGES AND CORALS 275

tentacle
mouth
endoderm
ectoderm
endoderm musculo-epithelial cell
mesentery sensory cell
mesoglea nematocyst
nerve net
mesoglea nerve net
ectoderm
enteron (b)

(a)

Figure 11.16 Morphology of Hydra: (a) general body plan, and (b) detail of the body wall.

relatively few different tissue types in a radial cnidarians exhibit both forms through their
plan. They are typified by the well-known life cycles, others only one. The Portuguese

hydra (Fig. 11.16). Although there are no spe- man-of-war, for example, is a spectacular and
cialized organs and only a few tissue types, scary colonial form with a medusoid module
they are more complex than the parazoans. for fl oatation and various types of polyps that
The group was, in the past, referred to as the help feeding, locomotion and reproduction.
Coelenterata, but because that phylum also As a whole the group is carnivorous, attack-

included the sponges and the gelatinous cteno- ing crustaceans, fishes, worms and even
phores or comb-jellies, the more restricted microscopic diatoms, with their poisonous
term Cnidaria is now generally preferred. stinging cells (cnidoblasts) – the reason they
Two basic life strategies occur (Fig. 11.17): are called “nettle-bearers”.
polyps are usually sessile or attached, although
some can jump and somersault, while medusae
swim, trailing their tentacles like the deadly Morphology: the basic cnidarian
and vicious snakes that adorned the head of
the mythical Medusa. Although medusoids The cnidarians are multicellular, having a
and polyps appear different, they are essen- single body cavity or enteron; the opening at
tially the same structures but inverted. Many the top (or bottom in most medusae), sur-

medusa
polyp

Figure 11.17 Cnidarian life cycles: generalized view of the life of the hydrozoan Obelia, alternating
between the conspicuous polyp and medusa stages.