A TUBULARIAN HYDROMEDUSAN 133 



animal kills its prey. Each one consists of a spiral, threadlike 

 tube, with several barbs at its base, which lies coiled within the 

 cavity of a cell called the cnidoblast. The cavity is filled with a 

 poisonous fluid ; its walls form an ovoid sac, of which the tube is 

 the very much elongated and invaginated outer end. A minute 

 spine projects beyond the free surface of the cnidoblast into the 

 water. When the surface of the ectoderm is irritated the tube is 

 evaginated and shot violently out, and the poisonous fluid con- 

 tained in the cavity of the nematocyst is injected into any animal 

 that may be struck. Look for nematocysts which have discharged 

 their spiral threads. 



Exercise 4. Draw the distal portion of a tentacle showing its cellular 

 structure; show the nematocysts at the end, including several 

 which have been discharged. 



Study the structure of the wall of the hydranth. It is made up 

 of an outer ectoderm and a much thicker inner entoderm, each 

 composed of a single layer of cells ; the inner ends of the ento- 

 derm cells are amoeboid and often flagellate, the function of the 

 flagella being to maintain in circulation the fluids in the gastro- 

 vascular space ; between these two layers is the thin, noncellular 

 supporting layer. Study the structure of the stem. Observe 

 its central cavity, which is a part of the gastrovascular space, 

 and the three layers just mentioned. In live specimens notice 

 the action of the flagella. Notice the cuticula which covers the 

 outer surface ; it is not found in the hydranth. This cuticula is 

 called the perisarc. It is a supporting structure and gives the 

 colony rigidity. 



Exercise 5. Make a drawing showing the cellular structure of the wall 

 of the hydranth and of the stalk so far as observed; carefully 

 label all. 



Special respiratory, excretory, digestive, and circulatory organs 

 are not present in the hydroid. Respiration and excretion are 

 carried on through the surface of the body wall. Digestion, 

 circulation, and absorption go on within the gastrovascular 

 space. The polyps feed upon small swimming animals, which 

 they kill or stun with their nematocysts and then swallow into 



