106 Ccelentera. 



The sexual generationj however^ is by no means invariaBly so 

 well developed. In very many HydromedussBj the medusoid buds 

 do not separate from the hydroids, but remain firmly attached. 

 Such sessile medusae (Fig. 61) remain in a more or less 

 incomplete condition; in some cases, an umbrella with marginal 

 tentacles develops, but the tentacles are small ; in others, the latter 

 are altogether absent, although the umbrella is well developed;; 

 in others, again, the umbrella itself is degenerate ; and lastly, it also 

 is wanting, so that the medusa consists simply of a mouthless 

 manubrium with, in the most degenerate forms, no cavity (Pig. 61, F; 

 Hydra). Without knowledge of the various transitional stages, 

 these small medusoid buds, which are never more than incipient, 

 would be regarded simply as organs of the polyp. The medusoid 

 bud, like the free-swimming medusa, always contains the sexual 

 cells, ova or spermatozoa, and new polyps or colonies arise from 

 these. 



In the Hydromedusse an alternation of generations as just described, generally 

 occurs ; there are, however, not a few exceptions. In various forms the ova do 

 not develop into polyps, hut give rise directly to medusae ; from these a hydroid 

 generation is altogether absent. Other deviations from the typical mode of 

 reproduction may be noticed, such, for instance, as the occun-ence, in some forms, 

 of an asexual multiplication of the medusse, which produce other medusae like 

 themselves, as buds from the manubrium or the margin of the disc. These 

 species also possess a hydroid generation, and the medusaj formed by budding 

 propagate sexually. 



The majority of Hydromedusse are marine, numerous examples 

 occurring in the northern seas. A considerable number both of 

 hydriform and medusiform persons are phosphorescent. Quite a 

 few, of which the genus Hydra is the best known, occur in fresh 

 water. 



1. Of the marine forms the Milleporidse deserve special consideration. 

 It has been mentioned above that stolons, suiTOunded by a continuation of the 

 chitinous covering of the polyp, usually arise from the lower end of the hydroid 

 stock, and often anastomose to form networks. These are sometimes of considerable 

 extent, and gives rise to a large number of small polypoid stocks or isolated 

 polyps. In the genus Millepora and allied forms the chitinous case is 

 calcified, and since new stolons are constantly being formed above the older 

 ones, the soft parts of which die away, these animals build regular coral-like 

 colonies, sometimes of considerable size. The outer layer is made up of 

 living stolons, from which the polyps arise, whilst the inner portion of the coral 

 consists of the calcified walls of dead stolons. The Milleporidse, which occur 

 exclusively in warm seas, play a not unimportant part in the formation of coral 

 reefs. 



2. The Fresh -water Polyp (Hydra), a small, elongate, solitary form, 

 without a chitinous cuticle; round the mouth, is a circlet of long tentacles 

 (4 — 10), which, like the whole body, contract vigorously when irritated. The 

 animal often remains for a long time in one place, e.g., with the lower end attached 

 just below the surface of the water ; but it is able to crawl about like a leech. 

 If an animalcule comesin to the vicinity of an attached Hydra, it is seized by the 

 tentacles, paralysed by the nettle-cells, and conveyed to the mouth. Hydra has 



