67 



three sorts of fever are caused by three distinct species of 

 parasite ; the benign and intermittent tertian and quartan 

 a,gue are produced respectively by Plasmodium (syn. 

 Hcemamo&ba) vivax and malarice, and the pernicious estivo- 

 autumnal fever or Tropical Malaria is caused by Plas- 

 modium falciparum. In these fevers the parasite is trans- 

 mitted by mosquitoes of or allied to the genus Anopheles. 

 Similar fevers occur in other animals, e.g., the rapidly- 

 fatal Texas Cattle-fever caused by Piroplasma(syn. Babesia). 



PHYLUM PORIFERA (SPONGES). 



What are Sponges ? 



They are (in the adult state) fixed sedentary animals 

 of plant-like appearance and aquatic habit. Most are 

 marine. 



The general structure of the body is a branching in- 

 ternal system of canals or waterways with numerous 

 openings on the outside. The water passes in through 

 countless minute inhalent pores (hence the term Porifera) 

 and, circulating freely throughout the body, passes out 

 ^kt one or several large exhalent openings, the oscula. 



Sponges are two-layered (diptoblastic) Metazoa. The 

 outer or dermal layer consists of flattened (epithelial) 

 cells, and there, is a middle jelly between it and the inner 

 or gastral layer, which is composed of flagellate collar- 

 cells or choanocytes that keep the water currents going 

 and secure the food. 



Spicule formation is a feature of most Sponges. The 

 spicules may be limy or siliceous and of various forms. 

 In the Hexactinellidse the glassy spicules are felted to- 

 gether into a supporting skeletal framework of exquisite 

 beauty (e.g., the " Venus Flower-basket " of Euplectella). 



