Page:Popular Science Monthly Volume 1.djvu/273

Rh the sea-water passes freely, and carries the digested food through the internal cavities. There is no other circulation of fluids;

Fleshy folds or partitions extend from the upper to the lower end of the animal, and give to the polype the appearance of a little balloon of tissue-paper crumpled up. These radial folds in the animal's structure are illustrated in Fig. 3, page 260, which is an ideal sectional view of one of this class of animals. The radiating partitions or folds are seen to be arranged in pairs. In the coral-secreting polypes these pairs are six or five, or multiples of six or five. The space between each of these partitions opens into the tubular tentacle at the top of the animal. The tentacles of the polypes moving freely and with

considerable muscular strength, seem sufficient to supply the animal's wants, but it has a formidable armature in the stinging barbs which cover its tentacles, mouth, and stomach, and which produce torpor and death in any small animal brought in contact with them. These are called lasso-cells. The cells in which the lasso or barbs are located measure from $1⁄350$ of an inch to $1⁄5000$ of an inch in length. From these the lasso is projected, inflicting, in some species, upon even a human hand, painful and serious injury. Owing to this peculiarity of certain jelly-fishes, they have been appropriately named sea-nettles.

Between the fleshy partitions of the polype's body are thin, stony plates. These, with the other hard portions, make up the coral skeleton, and are wholly secreted by the polype. Fig. 4, page 260, represents a group of polypes (Phyllangia Americana) from the West Indies, and illustrates well the radial structure of the cells.

The secretions occur around and underneath the polype, never in its interior, which would interfere with its functions. It constructs its skeleton by secretion, as an oyster does its shell, or as the tissues of a