HYDROID. 



OBELIA OR CAMPANULARIA. 



THIS hydroid has about the same mode of life and lives 

 in the same places as Pennaria. 



With a lens examine a colony making out the stem, branches 

 hydrorhiza, and hydranths. Compare with Pennaria on 

 these points. Make a drawing of the colony showing its 

 habit of growth. 



With the compound microscope examine the colony more 

 carefully and discover whether there is a perisarc as in 

 Pennaria. Does it differ from that in the other hydroid? 

 In these hydroids the perisarc at the end of each branch 

 widens out to form a funnel-shaped enlargement (the hy- 

 drotheca) which may entirely enclose the hydranth. Why 

 are some hydranths entirely enclosed within hydrothecse, 

 while others extend beyond this? The perisarc around the 

 stem, and around the hydranth as well, is of a horny con- 

 sistency and has been secreted by the ectoderm cells of the 

 coenosarc. When this was taking place the ectoderm was 

 in contact with the perisarc. Examine the colony closely 

 and find different stages in the formation of a hydro theca. 



Compare the hydranth with that of Pennaria in size, 

 shape and structure. How many tentacles are there? How 

 are they arranged? Are they alike? Note the mouth, 

 which is at the end of an enlargement, the proboscis or 

 hypostome. How is the hydranth supported in the hy- 

 drotheca? 



