PRIMITIVE ANIMAL FORMS 117 



exactly like the parent. The new hydra detaches itself and leads 

 an independent life such as a slip taken from a plant would do. 

 At a mean temperature of some twenty degrees, a well- 

 nourished hydra buds with great activity ; the hydra born of 

 these buds detach themselves very slowly, and only after having 

 produced buds themselves. In this way Trembley obtained a 

 hydra w r hich bore nineteen others in three different generations. 

 What is exceptional in the case of the ordinary Hydra becomes 

 the normal condition in the majority of the innumerable marine 

 species which, together with this creature, form the main group 

 of Hydroids. Their bodies are generally supported by a thin 

 covering of horny consistency forming the polyp capsule. 

 One of these Hydroids, Cordylophora lacustris, has succeeded in 

 acclimatizing itself to freshwater, and can be obtained in the 

 Seine. The Hydroids, fixed like plants, develop like them by 

 budding laterally, and ramifying ; they take on the appearance 

 of small shrubs whose branches consist of single hydroid 

 Polyps, just as the primitive plant was formed of leaves. The 

 polyps, by remaining associated, have constituted a new 

 organism, which is to each of them what a rose-bush is to its 

 leaves, and what the polyp itself is to the plastids of which it is 

 composed. It is formed by the same mechanism — an association 

 of like parts, each capable of leading an independent existence, 

 but which lose part of this independence by reason of their 

 association. 



Let us state at once that the mechanism we have seen at 

 work in the Vegetable Kingdom is also usual in the Animal 

 Kingdom. Thus, we ought to give a name to the organic forms 

 corresponding to the successive stages of this complication. 

 We have called plastids the simplest of these living elements, 

 which for a long time were called cells owing to incomplete 

 observations by the early histologists. We have described 

 as merids the organisms resulting from the association of 

 plastids. Hydras are consequently merids. An association of 

 merids we shall call zoids, and, as we shall also meet with 

 associations of zoids, we shall particularize them as denies. 

 These terms suffice to express all the stages of organic evolution. 

 Their brevity allows us to use them as suffixes in com- 

 pounds ; for example : spongomerid, hydromerid, bryomerid — 

 spongozoid, hydrozoid, bryozoid — spongodeme, hydrodeme, etc. 

 It sometimes happens that groups are formed within a deme 



