ORGANIC STRUCTURE: MORPHOLOGICAL 35 



whole sponge is essentially a congeries, or " colony," of such 

 collar-cells. Apart from specialized reproductive cells and a 

 supporting system of spicules, or fibres, there are no true tissues. 



QscuJbuLrri 



» 



Flagellate, 

 chamber 



Inhjcdent 

 '^pores 



To OscuIujTh 



'FhxgellwTi 

 CoUar 



Cell 



hodxf 



Fig. 4. 



I, A very simple sponge ; 2, diagrammatic section of the wall of such a simple sac-like 

 sponge ; 3, two of the characteristic, sponge " Collar-cells." 



\\c. The Hydra Type. Here we may have solitary indivi- 

 duals that are fixed to the stones, weeds, etc., in fresh or marine 

 waters (Hydra, Sea-Anemones), or solitary, pelagic (drifting) 

 forms (Medusae or " Jelly-fish), or colonies of individuals having 

 no particular arrangements. The structure is as follows : 



^Mouth^ 



£vuoL 



Ectodjenrc'- 

 -EruioderTTi-- 

 -MesodLerjTi --^3; 



EncLoderTTi 



^i^e of Attachment 

 I ^ ^ 



Fig. 5. — The Hydra Type. 



Essential structure of the solitary forms referred to above, and of the Zooids, 

 or segmental units of the Racemose type referred to below. 



I, A Hydra ; 2, diagrammatic section through one of such Zooids, or units ; 3, section 

 through the wall of a Hydra — this represents also the essential structure of the wall of a sea- 

 anemone, or medusa. 



The essential structure of a Hydra, or Medusa, or of a Sea- 

 anemone, as well as of each of the zooids of a very large group 



