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 
