THE HYDROIDS AND HYDROMEDUSJ]] 91 



tiniious throughout the branched stem and its rootlike extensions, the 

 hydrorhizae. 



(d) The entire colony is protected and supported by a thin, trans- 

 parent covering, the perisarc, which is not present in the hydras. The 

 perisarc of the stem is continued upward to form a cup-shaped hydro- 

 theca about each hydranth. Notice the shelf-like expansion of the 

 hydrothcca on which the hydranth rests, the ringed form of the peri- 

 sarc just below the hydrotheca, and certain places where the perisarc is 

 greatly thickened. Do you find places where the ectoderm of the 

 coenosarc is thickened? Explain. Draw a single hydranth, on a large 

 scale, including its connection with the upright stem. 



Exercise 3. — Reproductive Individuals, the Blastostyles, and their 

 IMedusse. 



(e) The blastostyles above noted are without mouths or tentacles. 

 How are they nourished? The covering of a blastostyle is the gono- 

 theca. How does it differ in shape from the hydrotheca? Attached to 

 the blastostyle are numerous rounded bodies, the medusa buds. Each 

 medusa bud becomes a medusa, or jellyfish, which is detached from the 

 blastostyle and escapes into the water through an opening at the end 

 of the gonotheca. The medusa may be considered a third type of indi- 

 vidual of the colony, but unlike the other two types, hydranths and 

 blastostyles, it is not a permanent member of the colony. What proc- 

 ess in the hydra is comparable with the detachment of the medusa 

 from its parent hydroid colony? Draw a single blastostyle on a large 

 scale, showing its connection with the main stem and also the medusa 

 buds in several stages of formation. 



(f) Examine demonstrations of the medusae of the obelia in the 

 stages just after detachment, when they are swimming freely in the 

 water. In the later stages of the medusae, reproductive organs appear, 

 as may be seen in a large hydroid medusa such as the species Gonione- 

 mus murbachii. Draw the medusa of Obelia. 



Exercise 4. — The Life-cycle. 



(g) The obelia colonies reproduce only by the asexual method of 

 budding. They never have gonads as do hydras. The medusse have 

 gonads and reproduce by syngamy, or the union of ova and spermato- 

 zoa in fertilization. The offspring of the medusae are not medusae, like 

 their parents, but are hydranths which, by extensive growth and bud- 

 ding, develop into a colony like the one just studied. The life-history 

 of the obelia thus exhibits an alternation of generations, since the 

 attached hydroid colony, reproducing by budding, alternates with the 



