n6 PRIMITIVE FORMS OF LIFE 



Thus, we become acquainted with three simple organic 

 types, and to these we shall give the name merids. These 

 merids may remain fixed or become, in this condition, 

 free. In the first case they become the respective starting- 

 points for the three great types of ramiferous organisms : 

 the Sponges, with complete mesoderm, the Polyps, with 

 no mesoderm, and the Bryozoa, with a mesoderm hollowed 

 out by a ccelom. 1 These three types must have been 

 simultaneously formed from the earliest times when life 

 existed on earth. 



The modern Sponges have the property of forming in 

 their tissues small mineral concretions, of a sharply determined 

 form, known as their spicules. These spicules may be siliceous, 

 or calcareous, or they may be replaced by fibres of a substance 

 analogous to silk, and known as spongin. The earliest Sponges 

 seem to have been provided with siliceous spicules bearing six 

 rectangular branches. Our seas still contain them, and they 

 constitute the family of Hexactinellidae. The Polyps and the 

 Bryozoa can also deposit mineral substance in their tissues, 

 but this is always calcareous. It was they that built the 

 calcareous deposits of former ages. The Bryozoa have had 

 but a humble destiny, while the Polyps, on the contrary, 

 have at all times played an important part ; thus, it becomes 

 necessary to detail with precision the relations they bear to 

 one another. 



One of the simplest forms in which they can be studied is 

 the freshwater Hydra, rendered so famous by the researches 

 of Trembley. It is, indeed, very difficult to imagine a more 

 primitive animal. It is a trumpet-shaped organism, six or 

 seven millimetres long, and attaches itself by its pointed end 

 to submerged leaves ; its orifice, serving at once as mouth and 

 anus, is surrounded by tentacles capable of seizing minute prey, 

 such as small crustaceans, worms, etc. After it has attained 

 a certain size the hydra ceases to grow along constant lines, but 

 produces laterally and in succession small protuberances 

 or buds, each of which develops in order to form a new hydra 



1 We may designate these principal merids as spongomerids, hydromerids, 

 and bryomerids. These latter differ from the merids that have given rise to the 

 Artiozoa, or, at least, to the Annelid Worms and their derivatives, only in the 

 fact that they have become fixed. This is one reason why the Bryozoa, 

 Brachiopods, and a part of the Gephyreans have been united in an artificial 

 group of Vermes, containing at the same time primitive and degenerate forms. 



