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 medusae, 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 gas tro- 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 medusae. The polyps are frequently 

 destroyed by frost or the beating of waves or by fishes, but new 

 ones quickly grow in their place*. 



The medusoid stage. The medusoids of campanularian hydro- 

 medusans are either sessile sporosacs or free-swimming medusae. 

 The medusae 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. 



