Page:Popular Science Monthly Volume 8.djvu/32

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of the loveliest of the hydroid family. The hydroids are among the coral-makers. The vast beds of millepores found about the Pacific islands and the West Indies are the work of an animal allied to coryne, one of the Tubularians. The chitinous investment of the Sertularians also forms membranous coral of considerable size and great beauty. It was some time, however, after the discoveries of Peysonnel, Jussieu, and Trembley, before the great authorities of the day, Réaumur and Linnæus, gave in their adhesion to the animal theory, and stamped it as correct. Since that day some of the world's greatest naturalists have made the study of the Hydroidœ their life-work, and have not felt it an unworthy occupation to be the annalists of this humble family.

The nomenclature of the hydroids is still so unsettled that we will avoid as much as possible the use of scientific terms in describing the different portions of the colonies and their respective functions, for it is here that naturalists differ, not in the names of the varieties.

The hydroids measure from a few lines in height to several feet. Dana mentions an East Indian species which grows to the height of three feet; while Semper describes a gigantic Plumularian, which forms submarine forests extending over great areas of sea-bottom, and growing as high as six feet. The stems, he says, sometimes measure an inch in diameter at the base. Tubularia indivisa grows to the height of about ten inches.

The Hydroidœ are divided into four families: Tubularinœ (Figs. 3, 4, 5, 6), Campanularinœ (Fig. 10), Sertularinœ (Figs. 1, 7, 8, 9), and Hydrinœ (Fig. 12).

Every hydroid, however greatly the species may differ in external form, has a certain structural plan to which it adheres in all its modifications. The general type (Fig. 2.) may be simply described as an animal sac whose walls are composed of an inner and outer membrane. The outer wall corresponds to the skin, the inner to the lining of the stomach, in higher organisms. The simple elongated sac is not only the simplest form of hydroid, but is generally the earliest phase in the development of the more complicated forms.

The sac (Fig. 2) sends off branching processes, e e, and cœcal protuberances, d, throughout the extent of which the inner and outer membrane is continuous. Sometimes large numbers of these stems proceed from a basal net-work, the connection between every part of the animal colony being kept open through this basal reticulation, and the continuity of the two membranes being maintained intact. The basal portion, with the stems, branches, and the flower-and-fruit-like clusters, of this curious organism form the hydrasoma, as it is called by both Huxley and Allman.

The simple, sac-like form of the hydroid is the lowest term in a series which consists of an almost infinite number of terms. We find in this family the same orderly sequence which marks organic Nature