113 



2. In many instances the effect on the host is negligible ; 

 but serious injury or disease is often caused, for example, 

 by parasites infesting the blood, by a bladder- worm situated 

 in the brain, by toxins which the parasitic worms or Hel- 

 minthes produce. 



COELOMATA. 



What is a Coelome ? 



Distinct from but generally surrounding the enteron o r 

 food canal, there is (in higher Metazoa) a space enclosed 

 by a wall of mesoderm ; it is developed from two pouches 

 of the archenteron which become shut off, or from certain 

 cells which are detached from the endoderm and which, 

 by repeated division, form a hollow structure : that is the 

 coelome or true body-cavity (cf. Mesoblast or Mesoderm, 

 page 15, Part I., " Catechism" Zoology}. Its functions 

 are excretory and generative ; it contains a fluid in which 

 there are wandering amoeboid cells (amcebocytes), and 

 these and the cells of its lining remove waste from the 

 body-tissues. 



The renal organs of higher animals are largely derived 

 from its wall ; and the reproductive elements arise from 

 a portion of its lining. 



Mention the chief Structural Features which distinguish the 

 Coelomata from the Port/era, the Hydrozoa, and 

 the Actinozoa. 



The Coelomata have a coelome or true body-cavity, 

 and a layer of mesoderm between the outer ectoderm and 

 the inner endoderm. The embryos are triploblastic (three- 

 layered). 



PHYLUM ANNELIDA (SEGMENTED WORMS). 



Give the general Characters of the Annelida. 



The elongated body is divided into a series of com- 

 partments (segments or metameres), and is marked exter- 



