INTRODUCTION. 



The Hydroids are mostly minute creatures 

 and nearly all marine. The individual animal 

 is termed a Polypite. They rank in 

 organization just above the Sponges and 

 below the Sea-anemones and Coral-polyps, 

 to which they are closely allied. 



Their structure is simple. A sack-like 

 stomach, the only external aperture being the 

 mouth, around which are arranged tentacles 

 armed with poisonous darts or stinging cells, 

 for the capture of prey. 



The exterior of the body-wall is composed 

 of a cellular layer (Ectoderm), some of the 

 cells of which by extending and withdrawing 

 lobes, perform the office of muscles, of which 

 the Polypites proper are destitute. Within 

 this is a delicate non-cellular membrane 

 (Mesogiaea), the inner lining being composed 

 of a layer of cells (Endoderm), which have the 

 property of throwing out pseudopodia and 

 flagella, by means of which circulation of the 

 food particles is kej^t up. Some of these cells 

 contain pigment and secrete a digestive fluid. 



A very few^ Hydroids (ex. Hydra) lead a 

 solitary existence. The great majority, 

 however, by a plant-like process of continuous 

 budding form colonies, the members of which 

 are all organically connected, hy reason of the 



