PHYLUM CCKLESTERATA 7'.l 



the cci'iiosarc (both stem and Ixxly) <»f each ( )l»clia iiidiviilual in a 

 colony is continuous with that of tlic main stern and therefore 

 with every other indiviihial in the colony; also that there is a 

 cavity within the iivinjii; stems which is connected with the dig^'s- 

 tive cavity of each individual. Of what advantaRe is this 

 condition to the animal? Some di^iestion takes place in the 

 cavity of the stems as well as in the f^astrovascular cavity of the 

 feeding polyps but the predominating type of digestion in ( )l)elia, 

 as in Hydra, is intracellular. 



The perisarc, throughout, stains more lightly than the c(rnosarc 

 and is of a homogeneous, nature. Is it uniform in thickness on 

 both sides of the stem? Is this thickening the same in successive 

 segments? Note the nature of the perisarc surrounding the 

 stems leading to the polyps. 



A nutritive individual of an Obelia colony is similar in fimda- 

 mental respects to a Hydra; the body wall consists of two cellular 

 layers, the ectoderm and the entoderm, and the thin, ncm- 

 cellular mesoglea. Hj/postomc, mouth, and tentacles are in the 

 same relative positions and are readily recognized. Compare the 

 tentacles of Hydra and Obelia and determine the most funda- 

 mental differences. The perisarc, surrounding the nutritive 

 polyp, is expanded into a goblet-shaped structure, which, since 

 it harbors a polyp, is called a hydrotheca (meaning hydra case). 

 Look for variations in the thickness of the hydrotheca. Note the 

 centrally extending ledge at the base of the hydrotheca supporting 

 the body of the polyp. 



The reproductive polyps are elongate in form, ending in a 

 broad, flat disk with neither mouth nor tentacles. Tiie walls of 

 these individuals produce by budding, the third kind of indi- 

 viduals in the colony, the medu.sae bud.s. Observe carefully the 

 structure of one of these. The perisarc is vase-shaped around the 

 reproductive polyps and is known as the gonotheca. 



The medusa' buds, when mature, separate from the core oi the 

 reproductive polyp or l)lastostyle, pass out through an opening 

 in the end of the gonotheca, and lead a free life. They are to be 

 regarded, nevertheless, as sister individuals to the feeding and 

 reproductive |)olyps, which remain attaciied to the colony. 



Upon the plate of Obelia label details to which attention has 

 been directed in the preceding paragraphs. 



The Study of a Medusa. .\s .stated al)ov«', the medusa' buds 

 when fully grown break away from the blastostyle and lead a 



