LESS. XXVI GENERAL STRUCTURE 305 



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 animals which are either unicellular in 

 the strict sense, or non-cellular, or colonies of unicellular 

 zooids : they have been represented in previous lessons by 

 Amoeba and Protamoeba, Haematococcus, Heteromita, 

 Euglena, the Mycetozoa, Paramoecium, Stylonychia, Oxy- 

 tricha, Opalina, Vorticella, Zoothamnium, the Foraminifera, 

 the Radiolaria, Pandorina, and Volvox. The reader will 

 therefore have no difficulty in grasping the general features 

 of this phylum. 



The Cxknterata are the diploblastic anipials, and have 

 also been well represented in the foregoing pages, namely 

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

 anemones and corals also belong to this phylum, in which 

 also the Porifera or sponges were formerly included. 



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

 blage. They are all triploblastic, but while some are 

 coelomate, 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 

 example of each will show how they all conform to the 

 general plan of organisation 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. 



X 



