THE CCELENTERATA ^ 163 



some senses it is. By this means, of course, more individuals are 

 added to the colony, but new colonies are apparently only formed 

 by the process of sexual reproduction. When living, the hydranths 

 are in a constant state of motion, waving their tentacles about in 

 search of prey, expanding and contracting, so that some of the more 

 brightly-coloured allies of Obelia are most beautiful objects and, 

 looking as they do like hundreds of animated flowers, well deserve 

 their name of zoophytes, plant animals. 



The blastostyle is a hollow outgrowth of the coenosarc with an 

 internal cavity continuous with the gastro- vascular canal. It 

 corresponds with a drawn-out body of a hydranth, and has a knob- 

 like enlargement at its distal extremity in place of the hypostome. 

 No indication of mouth or tentacles is to be found, and it is quite 

 incapable of catching prey, so that it is dependent for its food on the 

 nutritive polyps. The relation between it and an ordinary zooid 

 is not so obvious in Obelia as it is in certain other members of the 

 same class, irr which the medusoid person is actually budded off from 

 an ordinary hydranth. We therefore regard the blastostyle as a 

 highly modified and degenerate polyp, specialised for the production 

 of medusoids. As has been noted, the blastostyle is contained in an 

 urn-shaped expansion of perisarc, the gonotheca, and the two 

 together are sometimes termed a gonangium. A succession of 

 hollow buds are produced by the blasostyle, and these gradually grow 

 into rudimentary medusoid persons which after a while become 

 separated off. They undergo a little further development in the 

 gonotheca, but ultimately the top of this ruptures and allows them 

 to escape. 



The free-swimming reproductive individual of Obelia, the medusa, 

 differs very considerably in appearance from either the other zooids 

 or from Hydra. It is a small transparent organism about 2 or 3 mm. 

 in diameter, and shaped like a saucer, with a short stout handle in 

 the middle. From its likeness to an open umbrella the outer, 

 convex, or aboral surface is termed the exumbrella, and it was by the 

 centre of this that it was attached to the blastostyle. The under, 

 concave, or oral surface is spoken of as the sub-umbrella, and bears 

 in the middle the short more or less rectangular handle, the nianu- 

 brium, on the distal extremity of which opens a cross-shaped mouth. 

 The rim of the umbrella is fringed with a row of delicate tentacles, 

 which may be as few as sixteen when the animal is first liberated, but 

 increase to more than a hundred with age. The tentacles are well 

 provided with batteries of nematocysts, which, in the case of some of 

 the larger allies of Obelia, are capable of stinging human beings so as 

 to cause considerable pain. At the base of each tentacle, where it 

 is fixed into the umbrella, it enlarges slightly, and is covered by a 



