A CAMPANULARIAN HYDROMEDUSAN 139 



colony lives upon small swimming animals, which the feeding 

 polyps kill or stun with their nematocysts, and then swallow into 

 the gastrovascular space. Digestion goes on within the feeding 

 polyps ; the products of digestion mingle with the water present 

 in the gastrovascular space and circulate throughout the colony. 

 The entire colony is thus nourished, and if conditions are favor- 

 able it will grow rapidly and produce a large number of medusae. 

 The polyps are frequently destroyed by frost or by 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 disk-shaped jellyfishes, about one eighth 

 of an inch in diameter, which may be found swimming in the sur- 

 face waters of the ocean. Place several in a watch glass of sea 

 water or, if they are preserved specimens, in alcohol. If they are 

 alive, observe the swimming motions. Note the radiate type of 

 structure. The body resembles an umbrella in shape, having a 

 convex and a concave side, and is bordered by a fringe of tentacles. 

 The former is called the exumbrella or aboral side, the latter, the 

 subumbrella or oral side. In the center of the latter side is the 

 proboscis-like projection called the manubrium, at the distal end 

 of which is the mouth. This opens into the gastrovascular space, 

 which comprises the space within the manubrium and also a sys- 

 tem of canals in the disk-like body. These canals consist of four 

 or more radial tubes, which extend from the base of the manu- 

 brium to the periphery of the disk, and are there united by a cir- 

 cular tube which runs parallel with the margin of the disk and 

 close to it. 



Count the marginal tentacles. At the base of certain of the ten- 

 tacles are minute sense organs, called lithocysts, which are prob- 

 ably organs of equilibrium. Find them. 



Near the middle of each radial tube notice a prominent swelling 

 on the subumbrella. These are the sexual glands and are special- 

 ized portions of the ectoderm. The sexes are separate in medusae, 

 but the sexual glands have the same appearance in the two sexes. 



Around the inner margin of the subumbrella, at the base of the 

 tentacles, is a muscular membrane extending toward the manu- 



