everywhere. While the ideal type is adhered to, and a morphological unity may be proved, yet there is an orderly and beautiful gradation, in which each form becomes more complicated than the form which precedes it.
The clusters of buds (Fig. 4), and closed or open flowers (Fig. 3), are really individual zoöids, bound into an organic unity by a basal reticulation. With a single exception, every hydroid, at some period of its existence, lives this social life, being united with a number of other individuals into a plant-like group, and is really only one of an assemblage of zoöids possessing a common circulatory and nutritive system, the individuals of which are in organic union with each other.
Fig. 6.—Medusa of Tubularia indivisa. (After it has permanently fixed itself.)
The zoöids springing from one common base are of two kinds, and perform for the community two special offices. The grape-like clusters contain the generative elements, both ova and spermatozoa, while the flowers provide for the nutrition of the whole colony. These zoöids, which each investigator names according to his peculiar theory of scientific nomenclature, we will call nutritive and generative buds; the nutritive buds being destined for the preservation of the colony, the generative for the perpetuation of the species. The attached extremity of the animal in the fixed, or its equivalent in the free, species is called the proximal end, and the opposite extremity, which bears the two forms of buds, the distal end of the hydroid. The terms upper and lower cannot be used, because some varieties grow erect, while others grow in an inverted position.
The nutritive buds consist of an open digestive sac (Fig. 2);
