166 INVERTEBRATE ZOOLOGY 



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 non-cellu- 

 lar supporting layer. Study the structure of the stem. Observe 

 its central cavity, which is a part of the gastro- vascular 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; carefully 

 label all. 



Study the structure of the medusoid buds. Notice the differ- 

 ence between the younger and the older buds. Compare the 

 young buds with the hydranth buds ; compare the oldest 

 medusoid bud with the structure of the medusa, as described 

 later on. 



Exercise 6. Draw the oldest and the youngest buds identifying 

 as many of the parts as possible. 



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 gastro-vascular space. 

 The polyps feed upon small swimming animals, which they kill 

 or stun with their nematocysts, and then swallow into the 

 gastro-vascular space. Digestion goes on within this space 

 and waste matters are ejected from the mouth. The products 

 of digestion mingle with the water present in the gastro-vas- 

 cular space and circulate throughout the colony, the internal 

 cavities of all the individual polyps of a colony being in 



