24 Laboratory Guide in Zoology 



II. — Mount the specimen on a glass slide and observe it 

 through the low power. Notice that the animal is 

 composed of two general parts ; namely : — 



a. A firm outer covering, called the perisarc. 



b. An inner protoplasmic part — the living substance. 



III. — Identify the following five general divisions of the 

 perisarc : — 



a. The root-like expansion, called the hydrorhiza, by 

 which the stem is attached to whatever object it 

 grows upon. 



b. The upright stem portion, forming the linear cylin- 

 drical part of the colony. This is the hydrocaulus. 



c. The branches that have an open. Conical, bell-like 

 projection on the outer ends, inside of which are 

 small hydra-like bodies with numerous tentacles : 

 the projecting parts of the perisarc are called hydro- 

 thecae, and the hydra-like bodies are called zooids or 

 hydranths. 



d. Branches like those just described, but closed at the 

 outer ends and having protoplasm inside : these are 

 young hydranths in process of development. 



e. Branches that have a club-shaped tip, the gonan- 

 gium, inside of which are small, rounded bodies, the 

 medusa-buds. ♦ 



Make a drawing showing each of these parts and 

 label them distinctly. 



IV. — Study the structure of the perisarc. Notice its thick- 

 ness. Is it perfectly cylindrical throughout its length } 

 If not, where are the variations .'' 



V. — Make a careful drawing of a single zooid or hydranth 



with the tentacles extended. Show these points : — 

 a. The relation of the hydranth to the bell-like projec- 

 tion of the perisarc, which is called the hydrotheca. 



