ORGANIC STRUCTURE: MORPHOLOGICAL 37 



animals which are colonies of zooids of the Polyzoan type. There 

 are worm-colonies, ascidian colonies, etc. In all such cases the 

 individual animal consists of the segmental zooids, which are often 

 all nearly of the same structure, but often also the zooids are 

 differentiated so that reproductive, nutritive, flotational, aggressive 

 and other functions are specially performed by them. The 

 zooids are always aggregated by some special substance, the horny 

 material, or chitin, of the zoophytes, the massive, stony lime of 

 the corals, etc. As a rule, also, the fleshy substances of all the 

 zooids are joined by filaments, as shown in No. 2 of Fig. 6. 



11^. The Echinoderm Type. This is best illustrated by the 

 familiar starfish. 



Fig. 7. 



I, A Starfish seen from its lower side. The numerous small circles represent the locomotory 

 organs, or " tube-feet " ; 2, the same, seen from the upper side and dissected to show how the 

 alimentary cavity extends radially into each of the arms. The other organs have also such a 

 radial arrangement. 



The sea starfishes, sea-urchins, sea-cucumbers, etc. (all the 

 fossil species being included), form a homogeneous, well-defined 

 animal type in which there is, in spite of apparent diversity of 

 appearance, an essential similarity in structure. In all forms 

 there is upper and lower, radial symmetry. The '' arms " of the 

 starfish, and the corresponding parts of the other forms, are 

 segments each of which contains a similar, or nearly similar set 

 of organs. Peculiar to the Echinoderms is the mode of locomotion 

 by the suctorial '* tube-feet." 



nil. The Worm Type. The earthworm. No. 2 of Fig. 9, 

 illustrates the type. The term " AnneUds," applied to many of 



