22 THE POPULAR SCIENCE MONTHLY. 



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, Reaumur and Linnaeus, 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 MydroidcB 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 descx-ibes 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 Hydroidoe are divided into four families: Tuhularinm (Figs. 3, 

 4, 5, 6), Campanularince (Fig. 10), Sertularinm (Figs. 1, 7, 8, 9), and 

 Hydrince (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 modi- 

 fications. 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 coecal 

 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 



