172 INVERTEBRATE ZOOLOGY 
note the outer cuticular covering, the perisarc. Note the action 
of the flagella in a live specimen. 
Exercise 6. Make a drawing showing the cellular structure of 
the wall of the hydranth and of the stem. 
Study a blastostyle. We note that it is a cylindrical object 
enclosed within its transparent gonotheca. Budding out on the 
sides are the young disc-like meduse, those towards the free end 
being the largest and the oldest. The blastostyle has no 
tentacles and no mouth. It has an internal cavity which is 
a part of the gastro-vascular space of the colony, and within 
which the nutritive fluids circulate. 
Exercise 7. Make a drawing of a blastostyle. 
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 colony lives upon small swimming animals, which the 
feeding polyps kill or stun with their nematocysts, and then 
swallow into the gastro-vascular space. Digestion goes on 
within the feeding polyps; the products of digestion mingle 
with the water present in the gastro-vascular space and circu- 
late throughout the colony. The entire colony is thus nour- 
ished, and if conditions are favorable it will grow rapidly and 
produce a large number of medusz. The polyps are frequently 
destroyed by frost or the beating of waves or by fishes, but new 
ones quickly grow in their places. 
The medusoid stage. The medusoids of campanularian hydro- 
medusans are either sessile sporosacs or free-swimming medusa. 
The meduse are minute disc-shaped jelly-fishes, about one- 
eighth of an inch in diameter, which may be found swimming in 
the surface waters of the ocean. Place several in a watch-glass 
of sea water, or, if they are preserved specimens, in alcohol. 
