OBELIA J 



Asexual Stage 



1. Examine, with the dissecting microscope, a specimen of Obelia which 

 has been stained and mounted. Note the general form of the colony. 



2. Study the preparation with the low power of the compound micro- 

 scope. Note : (a) the upright stalk (hydrocaulus) with the side branches, 

 each of which bears either (6) a hydra-like nutritive polyp (hydranth) or 

 (c) a club-shaped reproductive polyp (gonangium). The stalk is com- 

 posed of (d) an inner, deeply staining portion (coenosarc), which is a 

 continuation of the same material in each polyp, and (e) an outer, trans- 

 parent exoskeletal sheath (perisarc) which encloses all parts of the colony. 

 Note that the perisarc expands into (/) a cup-shaped structure (hydro- 

 theca) surrounding each hydranth and into (g) a longer, urn-shaped 

 structure (gonotheca) which encloses each gonangium. Observe the 

 constrictions in the perisarc below each polyp. 



3. Study a hydranth and note : (a) the rody wall composed of an 

 outer layer of ectoderm and an inner layer of endoderm, both of which 

 are continuous with the coenosarc in the stalk ; (6) the central enteric 

 cavity, and (c) the tentacles which surround (d) the mouth. 



4. Study a gonangium and note that the club-shaped stalk (rlasto- 

 style) which, as in a hydranth, is continuous with the coenosarc, bears 

 numerous medusa ruds. Note the opening at the tip of the gonotheca 

 through which the mature medusa buds escape as free-living animals. 



5. Make a large drawing of an Obelia colony showing the structures 

 observed. 



1 B. pp. 68-73. 



271 



