HYDROIDS. 



23 



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 zooids, 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 zooids possessing a common circulatory and nutritive 

 system, the individuals of which are in organic union with each other. 



Fig. 6. Medusa of Titbularia indiyisa. (After it has permanently fixed itself.) 



The zooids 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 zooids, which each investigator names according to his peculiar 

 theory of scientific nomenclature, we will call nutritive and genera- 

 tive 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) ; 



