14 Invertebrata. 



that in point of complexity the sub-kingdoms overlap 

 each other. 



Sub-kingdom i. Protozoa ; the simplest animals, 

 which have neither body cavity nor nervous system, 

 and thereby differ from the other sub-kingdoms, which 

 possess at some time in their life-history a body cavity, 

 and which are collectively called Metazoa. 



Sub-kingdom 2. Polystomata ; sponges, which 

 have an internal cavity with one outlet and many in- 

 lets, bounded by a bilaminar wall. 



Sub-kingdom 3. Ccelenterata; jelly-fishes and sea 

 anemones, having a stomach cavity and a body cavity 

 as an outgrowth therefrom, and a radiate symmetry. 

 Month bordered by tentacles armed with thread cells. 



Sub-kingdom 4. Echinodermata ; star-fishes and 

 sea-urchins, also radiated, with a body cavity separate 

 from the stomach, a nervous system, and a system of 

 water-tubes which are agents in locomotion. 



Sub-kingdom 5. Vermes ; worms which are bila- 

 terally symmetrical, and composed of successive simi- 

 lar segments, with no jointed limbs, and with a water- 

 vascular system which has no locomotory function. 



Sub kingdom 6. Mollusca ; oysters, snails, &c., 

 possessing soft bodies enveloped in a leathery mantle, 

 no jointed limbs, a circulatory system, often an 

 external shell and an asymmetrical nervous system. 



Sub-kingdom 7. Arthropoda; crabs, lobsters, 

 spiders, and insects, which have bodies made up of suc- 

 cessive segments, with a symmetrical nervous system, 

 an external skeleton, chitinous or calcified, and jointed 

 limbs. Cilia are never present. 



Sub-kingdom 8. Vertebrata, including fishes, 



