56 LABORATORY DIRECTIONS 



cells. It is from these cells that the nettling cells are formed. 

 When fully formed the nettling cells migrate through the proto- 

 plasm of the epithelio-muscular cells until they come to lie at the 

 surface of the body where they are in a position to function. 



(d) The entoderm cells are much larger than ectoderm cells. 

 The protoplasm of these cells is usually rilled with rounded 

 masses of stored food material which has been taken directly 

 from the coelenteric cavity. These masses are frequently so 

 numerous as to obscure the nucleus. 



Make a drawing of 'a part of the wall of the body as seen 

 under high power. Individual cells must be at least three or 

 four times as large as in the low power drawing. Show the 

 structure very carefully in one or two cells of each kind. 



In your 200 mm. drawing of the entire animal show in out- 

 line the location and distribution of coelenteric cavity, ectoderm, 

 and entoderm. 



OBELIA 



Classification : Phylum Coelenterata, Class Hydrozoa, Or- 

 der Campanulariae-Leptomedusae. 



1. Obelia is a marine colonial coelenterate which becomes 

 attached to seaweed, piles, or other submerged objects. Study 

 prepared slide (or a piece of a colony on seaweed). Each in- 

 dividual of the colony is called a hydranth or zooid. Is there any 

 regularity in the arrangement of the zooids upon the upright 

 branches? Each branch usually represents but a part of a colony. 

 Several such branches may be united by a continuous root-like 

 hydrorhisa which is the part by means of which attachment is 

 secured. 



2. Make a simple diagram, using single lines to show the 

 arrangement of the parts of a colony. Structure is called for in 

 another drawing. 



