27 



CHAPTEE III. 



MESOZOA, SPONGES, CCELENTEEATES, AND ECHINODERMS. 



Mesozoa. 



The Mesozoa are a small group of invertebrate animals living as 

 parasites in frogs, earthworms, &c. Although they are of much 

 scientific interest, they are of none to the agriculturist. Their 

 chief importance to us is from their being intermediate between 

 the two great divisions of the animal kingdom. The principal 

 genus is a small parasite known as Dicyema. 



Spongid^ or Porifera. 



A sponge is a compound structure of true animal nature. It 

 is composed of contractile tissue, which is supported by a skele- 

 ton of hard spicules or fibres. In past ages sponges were thought 

 to be plants, but their true animal nature has long since been 

 demonstrated. The simplest form of sponge is represented by a 

 fixed cylindrical tube, with an exhalant opening, called the 

 osculum, at the free end. The contractile wall of the cylinder is 

 supported by rayed spicules, which may be calcareous or siliceous 

 and of very variable form: it is perforated by small pores, known 

 as inhalant pores, which lead into ciliated internal chambers. 

 In these ciliated chambers are found cells lining the cavities 

 peculiar to the Sponge. Such cells are called " collar cells," 

 each being provided with a long cilium and a distinct nucleus 

 in the lower part of the cell. The reproduction of sponges 



