32 THE FRESH-WATER POLYP 



colour of Hydra viridis is due to numerous green symbiotic Flagellates which 

 are present in the endod'erm cells. 



(c) The mesoglcea. 



This is a thin, structureless lamella, forming a sort of strengthening sheath. 

 On the outer surface a distinct longitudinal striation is formed by the muscle 

 processes of the ectoderm cells. On the inner surface transverse fibres occur, 

 which appear to be connected with the muscle processes of the endoderm cells. 



Make a drawing of a small part of the body- wall to show the layers and the 

 component cells. 



Put a drop or two of acetic acid and methylene blue at the edge of the cover- 

 slip, and observe the extrusion of the nematocysts. Notice that some of them 

 (small ones) are without barbs. Then gently tap the cover-slip, so as to break 

 up the dead Hydra ; and examine and draw the isolated cells. 



Two Ciliate Infusorians are often seen moving about on the skin of Hydra, 

 namely, Kerona, a commensal, and Trichodina, an ectoparasite. 



