A HYDROID (Clava). 



Clava is common along the whole New England coast, form- 

 ing small salmon-colored clusters on the rockweed (Fucus) 

 near low-water mark. The specimens should be collected and 

 allowed to expand in salt water. They are then killed with a 

 saturated solution of corrosive sublimate and passed through 

 successive strengths of alcohol, being finally preserved in 85% 

 Great pains must be taken to remove every particle of the 

 rockweed from the specimens or they will soon be so altered 

 as to refuse to stain. Each student should have a colony, 

 for study and also a slide of stained sections, cut parallel to 

 the long axis of the hydranth. These slides may be used year 

 after year. 



Examine a colony under the hand-lens, noting that it 

 is composed of hydranths, united at the base by a delicate 

 network (hydrorhiza). Do you find any perisarc (p. 123)? 

 What is the shape of the hydranth? Are the tentacles 

 arranged with any regularity? Where are the medusa- 

 buds ? Draw. 



In a mounted and stained slide of longitudinal sections 

 hunt with the compound microscope for a section passing 

 through the mouth. Make out the central digestive 

 cavity and in its walls see two distinct layers, an outer 

 ectoderm and an inner entoderm. Trace ectoderm and 

 entoderm to the oral end. After passing the mouth with 



which layer does the food come in contact? Find a sec- 



125 



