2 ENTOMOLOGY FOR MEDICAL OFFICERS 



The Enterozoa again are arranged in two series. The 

 one series includes animals like the polyps, sea-anemones, 

 and jelly-fishes (CCELENTERATA) in which the cavity enclosed 

 by the body-wall is the digestive-cavity ; the other series— 

 CCELOMATA — includes animals whose body-wall encloses a 

 body-cavity, or Calom, which has definite functions, and is 

 quite, distinct from the digestive-cavity that passes through 

 it or is suspended in it. 



Of the animals usually reckoned among the Ccelomafa, however, 

 there are some, such as the flat-worms, in which the body "cavity" is 

 parenchymatous. In some of the parasitic flat-worms, too, as also in 

 some of the parasitic round-worms, mouth and digestive-cavity do not 



exist. 



Protozoa, Porifera, and Coelenterata constitute three 

 several phyla of the animal kingdom. The Coelomata are 

 also split up into numerous phyla {cpvKov^s. clan, or stock). 

 Of the Coelomata phyla some are vigorous and prolific 

 stocks, while others are of the nature of stunted shoots or 

 insignificant relics. Of the great phyla the principal are : 

 (i) the Vertebrata which possess at least the prospective 

 elements of a backbone and skull ; (2) the MOLLUSCA, or soft- 

 bodied unsegmented animals like the snail and oysfer, which 

 are enveloped in a mantle and shell ; (3) the ECHINODER- 

 MATA, or radially-symmetrical Ccelomates, like the star- 

 fishes and sea-urchins ; (4) the ArthropoDA, or segmented 

 animals with firm exoskeleton and segmented appendages ; 

 (S) the Annelida, or segmented worms; (6) the Nemat- 

 HELMINTHES, or parasitic round-worms ; and (7) the Platy- 

 HELMINTHES, or parasitic flat-worms. The minor phyla 

 include a number of small, chiefly marine groups, such as 

 the Sipunculoidea, Chcstognatka, Polyzoa, and Brachiopoda ; 

 also the Rotifera, all of which are cousin with the Annelid 

 stock ; and the Enteropneusta, Tunicata (or Ascidians), and 

 Cephalochorda (Lancelet), which are satellites of the Vertebrate 

 system. 



On the theory of Evolution the members of all the phyla, 

 from the simplest Protozoon to the most specialised vertebrate, 

 are regarded as descendants of a far-remote ancestor, so that 

 the whole world is kin ; and the existing phyla are supposed 

 to represent the unperished limbs of a great genealogical 



