CCELBNTERATA. ,21 



mounted on slides for details, using 1., l.p., or h.p., 



as necessary. 

 Parts to identify : — 

 a. The branching stem. 

 h. The transparent sheath-like covering of the stem 



(perisarc). 



c. The tassel-like structures (hydranths) at the ends 

 of some of the branches. 



d. A vase-like body (gonangium) at the end of some 

 of the branches. This encloses a cluster of small 

 " buds " (medusa-buds). 



How much of this colony is a single individual ? 

 On what grounds can the hydroid claim cousinship 

 to the Hydra ? 



B. A Single Hydranth. (Corresponding to a single 



Hydra.) 



1. Are the tentacles arranged as in the Hydra ? 



2. Are all the tentacles of the same kind ? 



3. Can you see nettle cells on the tentacles ? 



4. Identify the mouth-opening surrounded by tentacles. 

 6. Does the perisarc surround the hydranth as well as 



the stem ? 



C. A Single Gonangium. 



This is a hydranth modified for reproduction. The medusa- 

 buds within the sheath of the gonangium breali free into the 

 water and by enlarging become medusae (see Number 4 in 

 Study III. on p. 23). These medusae produce eggs from which 

 develop new hydroid colonies like their grandparent form. 

 This manner of reproduction is called alternation of generations. 



1. Can a gonangium be said to have a mouth and tenta- 



cles in the same sense as an unmodified hydranth ? 



2. Are the medusa-buds larger at the distal or at the 



proximal end of the gonangium ? Explain why. 



