Page 253 - Introduction to Paleobiology and The Fossil Record

240 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

blastocel
early
blastocel embryonic mesoderm
gut enterocel Radial cleavage
Spiral cleavage cells

early
mesoderm embryonic
upper cells blastopore gut upper
view
view blastopore
Enterocoelus
Schizocoelus
(c) (d)

lateral
lateral view
view
(a) (b)
Trochophore-type larva Dipleurula-type larva
(e) (f)
gut
ectoderm endoderm

mesoderm
gut
endoderm
ectoderm
Triploblastic
Diploblastic (h)
(g)

Figure 10.5 Main invertebrate body plans and larvae: upper and lateral views of spiral (a) and radial
(b) patterns of cell cleavage; development of the mesoderm in the spiralians (c) and radialians (d);
diploblastic (g) and triploblastic (h) body plans and trochophore-type (e) and dipleurula-type (f) larvae.

Table 10.1 Key characteristics of the three main groups of animals.
Group Grade Symmetry Key character Larvae

Porifera Parazoan Bilateral and radial symmetry Collar cells Blastula larva
Cnidaria Diploblastic Radial symmetry Cnidoblasts Planula larva
Bilateria Triploblastic Bilateral symmetry Digestive tract Various types

rians. And finally the development of the segments; each segment possesses identical
celom or body cavity characterizes most of paired organs such as kidneys and gonads
the animal groups found as fossils. The celom together with appendages. The mollusks, on
usually functions as a hydrostatic skeleton the other hand, have an undivided celom situ-
and is related to locomotion. But the presence ated mesodermally and irregularly duplicated
and organization of the celom is not phyloge- organs.
netically significant; the celom has evolved The remaining bilaterians, such as the pho-

several times and in some groups, such as the ronids, brachiopods, bryozoans, echinoderms
flatworms, there are at least two types of and hemichordates have a celom that is

celomic cavities. divided longitudinally into two or three zones
The annelid worms and the arthropods each with different functions. Based around
have a celom divided along its length into this plan, animals with a specialized feeding