PORIFERA, HYDROZOA 235 



Order 2. — The Triaxonia are sponges vvilh large flagellate chambers, a 

 thin mesenchyme layer and triaxial silicious spicules. The latter may be 

 replaced by horny fibres or, occasionally, skeletal structures are wanting. 



Order 3. — The Tetraxonia have a complicated system of canals, small 

 flagellate chambers and a thick mesenchyme layer. The skeleton con- 

 sists of tetraxial or monaxial silicious spicules sometimes combined with, 

 or replaced by, spongin fibres. Euspongia is the commercial sponge and 

 Spongilla the fresh water sponge. 



524. The Cnidaria. — A great many Coelenterates are characterized by 

 the possession of peculiar organs, the cnidoblasts or nettling cells. These 

 occur in both ectoderm and entoderm but are often aggregated in certain 

 regions, as on the tentacles. The nettling cell contains a small capsule 

 which is filled wdth a fluid and contains a spirally wound thread. A sen- 

 sory point projects at the surface. When this is stimulated the capsule 

 bursts, the nettling thread is turned inside out and with it the fluid content 

 of the capsule is also ejected. The effect of this discharge is to paralyze 

 or kill the prey. Even the human skin is strongly irritated by the nettling 

 discharge of the larger Cnidaria and from this fact arose the name. The 

 Cnidaria are the Hydrozoa, Scyphozoa and Anthozoa. 



525. Class II. Hydrozoa. — This class is named after the 

 genus Hydra which is found in fresh waters very widely dis- 

 tributed. Practically all other Hydrozoa are marine. The 

 individual animals of this class are always small but many 

 species are colonial and the colony may attain to considerable 

 dimensions. There is usually a remarkable alternation of gen- 

 erations in which an asexual, fixed polyp form alternates with a 

 sexual free-swimming medusa. The polyp is in most essential 

 features like the hydra in form but the lower part of the column 

 is much elongated and slender thus forming a stalk. When 

 budding occurs the buds are not set free but remain attached 

 to the parent stem and thus is formed a colony. At certain 

 times another type of bud is formed. It difTers in form from 

 the parent polyp and is usually set free. This is the medusa. 

 Its principal axis is shorter than the radial axes so that it 

 assumes the form of a saucer or bell with a fringe of tentacles 

 around the edge. Near the margin of the bell on its concave 



