A TUBULARIAN HYDROMEDUSAN 167 



communication 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 medusas. The hydranths 

 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 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 are sporosacs, but 

 usually during the middle and last of the summer they detach 

 themselves from the hydroid and become medusae. Both con- 

 ditions may be found in the same colony and at the same time. 

 The medusoids of Bougainvillea are always free-swimming and 

 with other medusas will be found in the surface waters of the 

 ocean. They may be easily obtained by placing the live hydroid 

 colony in a small glass of sea water ; the medusas will be found 

 swimming about in the water. 



Place several medusas of Bougainvillea or of any other tubu- 

 larian hydromedusan 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 

 gastro-vascular space, which comprises the space within the manu- 

 brium 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 manubrium to the periphery of the body, and 

 are there united by a circular tube, which runs parallel with the 

 margin and close to it. 



