158 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 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 medusae. 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 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. 



