84 GENERAL BIOLOGY 



Make an enlarged drawing of an expanded and another 

 of a contracted hydranth. 



Examine a tentacle with the high power and find the 

 ectoderm and the entoderra layers, and the outlines of the 

 cells of these layers. How does the tentacle differ from 

 that of hydra, and from that of Pennaria? Are nematocysts 

 present? Where are they located? Make a drawing of 

 the basal portion of a tentacle showing the cell outlines. 



Look for branches which have an elongate, rather club- 

 shaped, enlargement of the perisarc. The ccenosarc within 

 will show no tentacles and no mouth. This is the gonangium, 

 or receptacle containing reproductive buds. The ccenosarc 

 forms a central core the blastostyle, which is a modified 

 hydranth body; from the sides of this are budded off small 

 medusae. . These medusae become free, escape from the 

 gonangium and swim away. Unlike Pennaria the medusae 

 of Obelia are not sexually mature at the time of escape, but 

 the eggs or spermatozoa are formed only after several weeks 

 of active life. The eggs -develop into new hydroids as in 

 Pennaria, the same alternation of generations being shown. 



Draw a gonangium. 



Campanularia differs from Obelia chiefly in its repro- 

 ductive process. In Campanularia no medusae are formed, 

 but within the gonangium eggs or sperms are produced. 

 Fertilization occurs within the gonangium and the egg 

 develops into a free swimming larva (planula) which escapes 

 from the gonangium. The planula fixes itself and trans- 

 forms directly into a polyp, which buds and produces a 

 colony. 



