vertebrate animal do its bones; an act wholly involuntary in both cases.
Therefore, when we speak of the labors or architecture of the coral animal, we do not imply outside mechanical work as the bee in constructing its comb, but simply the operation of a vital function. "This process of secretion," says Prof. Dana, "is one of the first and most common of those that belong to living tissues. It belongs eminently to the lowest kinds of life. These are the best stone-makers, for in their simplicity of structure they may be almost all stone, and still carry on the processes of nutrition and growth."
The young polype in the reef-building species arises by a process of budding from the parent animal. It was from this curious operation that early observers strengthened their argument in favor of the vegetable nature of corals. "The bud," says Dana, "commences as a slight
Fig. 7.
Brain-coral.
prominence on the side of the parent. The prominence enlarges, a mouth opens, a circle of tentacles grows out around it, and increase continues until the young finally equals the parent in size. Since in these species the young do not separate from the parent, this budding produces a compound group."
From this it is obvious that, while the polypes exist as individuals,
