Manual of comparative anatomy and physiology / by S. Messenger Bradley.

  • Samuel Messenger Bradley
Date:
1874
Catalogue details

Licence: Public Domain Mark

Credit: Manual of comparative anatomy and physiology / by S. Messenger Bradley. Source: Wellcome Collection.

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    It is perhaps possible to give a sketch of the evolution of animal forms in a few sentences which may be sufficient to illustrate the general doctrine. After the evolution of such simple animals as the Rhizopoda, which we have seen to be masses of unnucleated protoplasm, the first step would be for the sarcode to acquire a denser envelope. This change would be followed by the development of cilia from its surface to supplant, as locomotive organs, the pseudopodia vfhich would no longer be protrudible ; one part of the envelope would then become indented, to permit of the readier ingress of food, and so the Rhizopod would evolve the Infusorium. The steps are easy from the Infusoria to such simple Coelenterata as the Hydrse, which give us a clue to further progress. The anim^als now reached are cellular in constitution, and constantly develop congeries of cells, or buds, from the sides of their body, which may either remain attached to the parent organism, or may become detached, when they will themselves grow up into forms like those from which they sprung. If these buds remain attached to the parent they may nevertheless become matured, and in this manner we are able to understand the development of such compound animals as Salpidse or Spongidse, or the compound Actinozoa. Thus, e.g..^ such animals as Sponges are formed by an integration (by simple adhesion as it were) of a vast number of minute but independent organisms, which are loosely united, to form the entire colony, each organism sub- serving some function of the compound animal. These changes are simply due to a ]3rocess of gemmation, which perhaps may be made clear by the accompanying diagram, for it is easy to understand how such a creature as a might, by budding, evolve an organism like h. a Phalluda mentula. h Perophora—designed to show how a compound Ascidian, growing from a com- mon stolon, may be evolved from a solitary and simple form. Fig. 1. Diagram to illustrate the Evolution of Ascidioida. Now when each of these gemmiparously produced individuals,
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