210

14

The Nature of Living Things

Table 14.7 The major divisions (phyla) of animals

Phylum

CharacteristicSuperscript normal a^a

Examples

Porifera

No permanent tissue

Sponges

Coelenterata (cnidaria)

2 or 3 layers of cells

Nematode worms

Ctenophora

2 or 3 layers of cells

Comb jellies

Annelida

mesoderm has a cavity

Earthworms

Arthropoda (tilde four fifths∼⁴

5 ^{of all animal}

species)

Jointed limbs

Insects, crustaceans, arachnids

Mollusca

True coelom

Snails, octopus

Echinoderma

urchin-skinned

Starfish

ChordataSuperscript normal b^b

Backbone, skull

–

Superscript normal a^aTissue appears with the coelenterata, initially as two layers of cells—an outer (ectoderm) and an

inner (endoderm)—separated by a structureless jelly. In the more advanced exemplars, the third

layer of cells, the mesoderm, replaces the jelly. These are the three primary so-called germ layers

of cells, which further differentiate into more specialized organs. The main animal tissue types are

epithelial, connective, muscle, and nervous. The topology of the coelenterata is that of a simple sack.

The mesoderm cavity that appears with the annelida develops into the coelom of the mollusca [cf. the

main plant tissue types: epidermal, vascular, ground (subdivided into parenchyma (responsible for

photosynthesis (the mesophyll), storage, etc.), collenchyma (structural), sclerenchyma (structural,

without protoplasm; i.e., fibrous); meristematic ground tissue is responsible for growth]

Superscript normal b^bThe chordata (craniata) are subdivided into subphyla including the vertebrata, whose classes com-

prise the familiar agnatha (lampreys, etc.), fish, amphibians, reptiles, birds, and mammals

discovery is of great importance, because it allows perhaps any cell to be converted

into the equivalent of an embryonic stem cell, which is much more troublesome to

obtain directly (from an embryo).

14.9.2

Epigenesis

The fundamental problem of differentiation is that all of the cells have the same com-

plement of genes. How, then, can different types arise? Pluripotent stem cells can be

made to differentiate into neurons, for example, by exposing them to retinoic acid

(at a concentration exceeding a certain threshold). If the initially differentiated cells

then secrete a substance that blocks their as yet undifferentiated neighbours from

differentiating, a stable population of two cell types results. ⁶³ Differentiation is thus

seen to be a typical complex phenomenon (cf. Sect. 4.2). If all cells were at all times

identical, then, of course, differentiation could never occur. Even if all are endowed

with the same maternal substance that induces differentiation, however, provided that

the quantity of the substance is small enough for appreciable fluctuations in its con-

centration to occur (among, say, the 16 blastomeres), then they will not differentiate

63 Luthi et al. (1998).