The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

CHAPTER 4

LESS INDIVIDUALIZED ANIMALS

§ 1. A Preliminary Note on Individuality. § 2. Obelia. Sea-Anemones, Corals.

8 1 A Preliminary Note on Individuality

E are accustomed to think of ourselves

as individuals, for our bodies act and feel as single undivided things ; we are in this sense the units of which our species consists. Nevertheless we are also cell-communities, and these cells, in such circumstances as we have described in Book One, can behave with remarkable individuality and independence. We may indeed distinguish two entirely different grades of individuality—the conscious, actual individuality of our bodies and the potential, suppressed individuality of our constituent cells. Now, as we explore the realms of life farther and farther from our own phylum, this grading is less distinct. All the organisms that were examined hitherto are individualized communities of cells just as we are, but in some of them the organization of the community is a little less definite, less inflexible than it is in ourselves. The cells are not so rigidly subordinated, not so strictly specialized.

Perhaps the most independent cells that we possess, excepting, of course, our spermatozoa, are our white blood corpuscles, ranging more or less freely in our blood and lymph and playing the part of scavengers, consuming and destroying bacteria or other undesirable objects. ‘They are least subject to the disciplining influences that hold most of our cells in place and make them do special and limited activities at appropriate moments. Now, in many invertebrates freely moving cells of this kind play other parts in the economy of the organism, not merely absorbing undesirable matters. In the oyster, for example, the digestion and absorption of food is accomplished in part by creeping cells which elbow their way through the wall of the digestive tube into its canal; they fall upon the food, taking it into their own tiny bodies, and then they creep back through the wall of the intestine into the tissues of the oyster again. They even assist in transporting food round the body, for they creep everywhere and give up the particles that they have taken in wherever they are required. Further than this their liberties extend, for they can leave the body of the oyster altogether and prowl

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§ 3. Polyps, Jelly-fsh, § 4. Sponges.

about in the mantle cavity (the space between the creature’s body and his shell), where, presumably, they act as outposts against invading bacteria. In the earthworm there is a similar state of affairs ; creeping cells leave his body and wander about over his skin, scavenging and keeping it clean. Truly an astonishing degree of freedom for cells that are really parts of a body !

The more flexible organization of the lower animals is clearly seen in the reproductive processes. We ourselves have only one method of reproduction; we have definite cells set aside for the purpose, and the only possible way in which a mammal can originate is by the union of an ovum with a spermatozoon. Moreover, our developmental processes are nicely regulated; a particular group of cells in the embryo is delegated to form an arm, and that is the only way in which an arm can be formed ; if we lose an arm the part can never be replaced. In the lower vertebrates, on the other hand, the cells are not so rigidly specialized ; a newt, for example, can grow a new limb if one is accidentally lost. Lizards can grow new tails, and they make good use of their ability; if a lizard is pursued and finds itself hard pressed it deliberately breaks off its own tail, which, writhing aimlessly like the body of a newly decapitated man, distracts the pursuer while the rest of the lizard makes its escape. In those very lowly organized members of our own phylum, the sea-squirts, the tissues have a tremendous power of reconstitution. There are kinds of sea-squirt in which the animal can be cut completely in halves, and in which each half then grows the missing organs so that two individuals result. This power which ascidian tissue has of reconstituting itself is used for reproducing the species ; besides the normal sexual method the animals can reproduce asexually by sprouting out buds. There are sea-squirts, such as Doliolum, that bristle with buds as a pageboy bristles with buttons. Often, the individual sea-squirts formed in the latter way do not part company, but remain close together, so that by several asexual generations extensive groups or colonies of animals are built up.

As we pass from the sea-squirts to ourselves, we see an increase in the specialization