3o8 CHARACTERS OF THE HIGHER ANIMALS LESS. 



With a few exceptions, the discussion of which would be out 

 of place here, the vast number of animals known to us may 

 be arranged in one or other of these groups. 



The Protozoa are the unicellular animals : they have been 

 represented in previous lessons by Amoeba and Protamoeba, 

 Hsematococcus, Heteromita, Euglena, the Mycetozoa, Para- 

 mcecium, Stylonychia, Oxytricha, Opalina, Vorticella, Zooth- 

 amnium, the Foraminifera, and the Radiolaria. According to 

 many authors, Pandorina and Volvox are also included in 

 this group. The reader will therefore have no difficulty in 

 grasping the general features of this phylum. 



The Cxlenterata are the diploblastic animals, and have 

 also been well represented in the foregoing pages, namely, 

 by Hydra, Bougainvillea, Diphyes, and Porpita. The sea- 

 anemones, corals, and sponges also belong to this phylum. 



The Vermes, or Worms, are a very heterogeneous assem- 

 blage. They are all triploblastic, but while some are 

 ccelomate, others have no body-cavity; some, again, are 

 segmented, others not. Still, if the structure of Polygordius 

 is thoroughly understood, there will be little difficulty in 

 understanding that of a fluke, a tape-worm, a round-worm, 

 an earthworm, or one of the ordinary marine worms. 



Of the remaining four sub-kingdoms we have, so far, 

 studied no example, but a brief description of a single 

 typical form of each will show how they all conform to the 

 general plan of organization of Polygordius, being all triplo- 

 blastic and ccelomate. 



Under the Echinodermata are included the various kinds 

 of starfishes sand-stars, brittle-stars, and feather-stars, as 

 well as sea-urchins, sea-cucumbers, &c. A starfish will serve 

 as an example of the group. 



The phylum Arthropoda includes crayfishes, lobsters, 

 crabs, shrimps, prawns, wood-lice, and water-fleas ; scorpions, 



