CAMPANULAEIAN HTDEOID 93 



they entirely covered by the perisarc ? What 

 is the shape of that portion (hydrotheca) of the 

 perisarc which covers the body of the zooid? 

 Note the tentacles. How many has each hy- 

 dranth ? What is their shape ? Of what are 

 they composed? How are they arranged? 

 Note the groups of nematocysts or " lasso- 

 cells" on the tentacles. How are they ar- 

 ranged? Are the tentacles flexible or rigid? 

 If living hydroids can be obtained isolate some 

 of the nematocysts by crushing a tentacle un- 

 der the cover-glass, and note their structure. 

 Compare with Hydra. Examine also the 

 mouth at the outer end, or manubrium, of the 

 body proper. Compare several zooids to see 

 if the manubrium is flexible and the mouth 

 distensible. What is its structure? Does it 

 bear tentacles ? Examine the body of the hy- 

 dranth. What is its shape? Of what is it 

 composed ? What is the shape of the body 

 or digestive cavity? Is the body-cavity a 

 closed cavity ? How far does it extend into 

 the tentacles ? Do you ever in preserved spec- 

 imens find in this cavity small molluscs, crus- 

 taceans, etc., which have been captured for 

 food ? How does a hydranth compare in struct- 

 ure with a hydra ? 

 Make an enlarged drawing of a single hydranth 

 with its stalk, showing all of the structures visible. 



2. The reproductive zooids. — Position ? Shape ? 

 Are they covered by the perisarc? Compare 

 the shape of this portion (gonangium) of the 

 perisarc with a hydrotheca. Do they have ten- 

 tacles ? How do these zooids differ in internal 



