Page:Popular Science Monthly Volume 16.djvu/663

 of the wall of the body, that vary in number with the species, or sometimes with individuals, but are constant for each during the great part of its life. The other is formed of a stomach-like sac, open at bottom, around which are hollow tentacles, which often increase in number with the age of the Polyp (Fig. 10). These tentacles, which are free at their extremities, and united at their bases to form the wall



of the Polyp's body, open inward like the stomachal sac into a great cavity, the circumference of which is divided into cells by the soldered walls of two neighboring tentacles. On the partitions of these cells, and so within the body, the reproductive apparatus is developed; while in Hydroid Polyps it is generally on the exterior in the form of a bud. This type of structure is much more complex than that of the Hydroid Polyp, which is well represented by the Stylasteridæ. In their colonies we find the polymorphism of the Hydroida, and also the nourishers, purveyors, and reproducers. Among the Spinipora, Sporadopora, Pliohothrius, Errina, these different sorts of individuals are perfectly independent of each other: a simple vascular network distributes among them the food seized by the hunters and elaborated by the nourishers.

But with the Millipores the nourishers are the most important members of the colony, as they prepare all the nourishment, drawing around them the hunters and reproducers, but without establishing any more intimate relations. With the Astylus, the Stylaster, the Cryptohelia, this movement of concentration around the nourishers becomes pronounced; a space forms underneath; the tentacles, rendered useless by the neighborhood of the hunters, disappear, and nothing remains but a digestive sac around which the hunters perform functions exactly like those of the tentacles of a Coralarian Polyp. Each system has now a decided individuality. Another step, and the hunters, from being distinct throughout their whole length, grow together at the base and interlace with the digesters, and the reproducers follow in this movement. These different parts are, thenceforward, too near together to require a special vascular system; the vessels which unite them are simple perforations of their wall which open in the space just below the digesters, and into which the reproducers