GENERAL ZOOLOGY 



along the surface of attachment. In the fully developed colony there are two 

 types of individuals: (1) hydranths, which have mouths and tentacles, and 

 coelenterons continuous with the tubular cavity pervading all the stems of the 

 colony; and (2) gonanths, modified polyps without either mouths or tentacles, 

 consisting chiefly of a central, rod-like blastostyle upon which are formed the 

 buds which become free-swimming medusae. Both hydranths and gonanths 

 are surrounded by appropriately shaped secreted containers, known respec- 

 tively as hydrothecae and gonothecae, which are specialized portions of the 

 perisarc surrounding the entire colony. The living material, continuous 

 throughout the colony, constitutes the coenosarc. 



The obelia colony, with its clearly marked dimorphism ("two forms"), thus 

 arises by a process of repeated budding involving also the differentiation of 

 the two types of polyps. The medusae, or jellyfishes, which constitute the 

 products of asexual reproduction in the gonanths, are specialized in yet an- 

 other direction — for swimming and sexual reproduction. After their release 

 from the parent gonanth they reach sexual maturity as males or females, 

 having either testes or ovaries. The gametes produced in these gonads are 

 released into the water, where fertilization occurs. The resulting zygote 

 develops into a ciliated, solid-bodied, two-layered, free-swimming stage, the 

 planula, which settles to the bottom and transforms into an attached polyp 

 from which a new colony is formed. In Obelia, sexual reproduction is carried 

 on only by the medusa generation, and asexual reproduction is limited to the 

 hydroid generation. Thus, we may say that there are two generations which 

 alternate; the term metagenesis is sometimes applied to such alternation of 

 generations. The medusae formed by hydroid colonies, and other similar 

 medusae, are called hydromedusae to distinguish them from the larger, more 

 specialized jellyfishes belonging to the class Scyphomedusae. 



Not all the genera of the Hydrozoa exhibit alternation of generations. 

 Some colonial forms develop reduced or degenerate medusae, often called 

 gonophores, which are never released but which develop gonads and produce 

 gametes while still attached. In other genera such fixed medusae grade into 

 special gamete-forming structures termed sporosacs; in still others, as in the 

 solitary hydras, the gonads develop directly on the polyps, without a trace 

 of the medusoid generation. In the opposite direction lie such forms as 

 (1) Gomonemus, with a large, free-swimming medusa produced by a minute 

 polyp which scarcely buds except to generate the medusa; and (2) Liriope, in 

 which the planula develops directly into the medusa without a trace of the 

 polyp. The entire assemblage of hydrozoans may be arranged in a regular 

 series, with exclusively polypoid forms like Hydra at one extreme, and ex- 

 clusively medusoid forms like Liriope at the other. In such a series Obelia, 

 with its polypoid and medusoid generations about equally represented, lies 

 in the center. It may be suggested that this kind of a series probably repre- 

 sents an evolutionary sequence, but the direction of evolutionary change is a 

 matter of conjecture. It seems illogical, however, to consider Hydra a primi- 

 tive stem form; it is one of the very few fresh-water hydrozoans, is non- 

 302 



