A MANUAL OF ZOOLOGY 11 



Cnidoblasts or thread cells wiUbe found among the cells 

 of the ectoderm. They contain nematocysts or capsules 

 In which is a coiled thread. The cnidoblastg are terminated 

 by a sharp point, the cnidocil or trigger hair. Are these 

 cells uniformly distributed over the tentacles? Study the 

 ectoderm and the entoderm for any difference in shape 

 or size of the cells. What is the extent of the entoderm? 

 Make a drawing of a cross section through the body and 

 one through a tentacle. Does the hydra have any special 

 sense organ? Run a drop of dilute acetic acid under the 

 cover and note the effect on the nematocysts. Find a 

 specimen that is budded and see what relation exists 

 between the body cavity and the bud cavity. Make draw- 

 ings to show parts referred to above. Under favorable 

 conditions ovaries and testes may be found on th^ same 

 specimen. The former are located near the base of the 

 animal, the latter as^ small knob-like swellings near the 

 tentacles. Spermatozoa may be demonstrated by crushing 

 a specimen under the cover. If found, describe themr 

 2. Campanularian Hydroid 

 Alcoholic material a,s well as permanent stained 

 mounts will be furnished. If possible get a specimen 

 attached to some support. Examine the attached end, 

 hydrochiza; the stalk, the hydrocaulus; and the branches 

 with the hydranths. Would this cluster be considered a 

 colony or a single animal? Note any differences in the 

 hydranths. How are the branches arranged? Fifld a trans- 

 parent covering, the perisarc. What is its extent? Is there 

 any difference in arrangement at any point? Under this 

 layer find the fleshy portion, the coenosarc. Compare with 

 the hyclra for the layers: the entoderm, the ectoderm, and 

 the supporting layer. What is the extent of the body 

 cavity? Look for two forms of zooids: the feeding, and the 

 reproductive zooids. Note the extent of the perisarc in 

 the former. Find the nematocysts (lasso cells) on the 

 tentacles. How many tentacles are there? Compare them 

 with those of the hydra. Note the mouth at the outer end, 

 or the manubrium. How do the reproductive zooids com- 

 pare with the ones just studied? Look along a central 

 stalk (blastostyle) for the medusae which are finally to 

 be shed. Make an enlarged drawing of the entire animal. 



