134 



PHYLUM CCELENTERATA 



the gastrovascular 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 gastrovas- 

 cular space and circulate throughout the colony, the internal 

 cavities of all the individual polyps of a colony being in com- 

 munication with one another. When a colony is well fed it grows 

 rapidly, new hydranths bud out from the stalk, and medusoid 

 buds appear and grow into medusae. The hydranths are fre- 

 quently 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 tubularian hydro- 

 medusans are either sessile sporosacs or free-swimming medusae. 

 Pennaria produces both kinds. During a greater part and in 

 some cases the whole of the year they remain attached to the 

 colony and are thus sporosacs, but usually during the middle and 

 last of the summer they detach themselves from the hydroid and 

 become free-swimming medusae. Both conditions may be found 

 in the same colony and at the same time. The medusoids of 

 Bougainvillea are always free-swimming and with other medusae 

 will be found in the surface waters of the ocean. They may 

 usually be easily obtained by placing the live hydroid colony in 

 a small glass of sea water ; the medusae will be found swimming 

 about in the water. 



Place several medusae of Bougainvillea or of any other tubu- 

 larian 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 is bell- 

 shaped, having an outer convex and an inner concave side, and 

 on its margin are tentacles (in the medusa of Pennaria they are 

 rudimentary) . The convex side is called the exumbrella or aboral 

 side, and the concave, the subumbrella or oral side. In the center 

 of the latter is the proboscis-like projection called the manubrium, 

 at the distal end of which is the mouth, surrounded by short 

 oral tentacles. The mouth opens into the gastrovascular space, 

 which comprises the space within the manubrium and also a 

 system of canals in the bell-shaped body. These canals consist 

 of four radial tubes, which extend from the base of the manu- 



