HYDRO ID 81 



the points of difference. A mouth is present at the tip of 

 the hydranth, but, unless open, will not readily be found. 

 Is there a cavity present as there was in hydra? Does it 

 extend into the stem and the tentacles? 



Make an enlarged drawing of a single hydranth with a 

 portion of the adjacent stem. 



With the high power examine the different tentacles of a 

 hydranth with care and make out the ectoderm and the 

 entoderm, and the boundaries of the cells. Are the tentacles 

 alike in the arrangement and relative size of the layers and 

 of the cells? Are they hollow as in Hydra? Look for 

 bladder-like, oval, transparent cells in the tentacles (the 

 nematocysts or stinging cells). Note their size and arrange- 

 ment. 



Make drawings of portions of the different tentacles as 

 seen under the high power. 



3. Reproduction. Reproduction in, Pennaria, and in the 

 hydroids generally, is of two kinds, asexual and sexual. 



(a) Asexual. The entire colony is produced by budding. 

 Look for buds on the sides of the stem, just below the hy- 

 dranths. These buds produce new hydranths, and there- 

 by increase the size of the colony. Other buds are formed 

 on the sides of the hydranths, these are called medusae and 

 when they are full grown they separate from the hydranth 

 and float freely in the water. They have the power of 

 locomotion and swim about as distinct individuals. If 

 possible make out the structure of the larger ones. 



(6) Sexual. The medusae formed by budding from the 

 sides of the hydranths are sexual individuals, and they pro- 

 duce either. eggs or spermatozoa. The eggs and the sper- 

 matozoa are ripe when the medusae are liberated, and are 

 cast into the water where fertilization occurs. The fertilized 

 egg develops into a free swimming larva which after a time 



