Zoology : an elementary text-book / [Sir A.E. Shipley].

  • Arthur Shipley
Date:
1920
Catalogue details

Licence: Public Domain Mark

Credit: Zoology : an elementary text-book / [Sir A.E. Shipley]. Source: Wellcome Collection.

    49/780 (page 25)
    parts in a definite order, but upon this principle of the repetition of similar parts the bodies of the most complicated animals are built up. It is no doubt fundamentally the same thing as repro¬ duction, only that the various units which are produced, instead of separating from each other and leading separate existences, remain connected, and, as we say, are co-ordinated to form an individual of a more complex kind. In Polystomella the various parts are called chambers; a name which properly belongs, and was first applied to, the segments of the shell enclosing them. It is worthy of note that it is only the protoplasmic body of Polystomella which shows this composition out of definite units arranged in a definite order; there may be one large nucleus or a considerable number of smaller nuclei, but they are not arranged in correspondence with the chambers. The Thalamophora, which, like Polystomella, have compound shells, are very numerous and include an immense variety of forms, the variety being brought about by differences in the number of chambers and the way they are arranged in series. They almost all live in the sea; some, like the two we have described, creep about amongst the sand and debris at the bottom of pools or other places where the water is quiet; many others float at the surface of the ocean, the protoplasm which clothes the outside of the shell having numerous vacuoles filled with fluid probably less dense than the sea-water, and thus serving as floats. In such inconceivable myriads do these floating Thalamophora exist, that their empty shells form thick banks of impalpable white chalky miid at the bottom of the ocean, and the familiar white chalk of our English cliffs and hills is largely made up of the shells of Thalamophora. When such a form as Polystomella is about to reproduce by fission the protoplasm emerges from the old shell and divides repeatedly into a number of pieces .each as large as the central chamber of an ordinary individual. These then secrete round themselves shells and begin to bud off new chambers and gradually acquire the size and shape of adults. When on the contrary Polystomella forms gametes, the nucleus seems to break up into a chromidium like that of Arcella, which is dispersed throughout the protoplasm, and the protoplasm whilst still within the shell divides into a large number of small rounded pieces, each of which contains one of the minute fragments of the nucleus. These rounded bodies are gametes; they acquire hair-like projections called f 1 a g e 11 a which can be moved to and fro and by means of which they can