474 ACALEPELE ; MEDUSA. 



probably replace them in function. The most curious modifica- 

 tion, however, is that which is displayed to us in the Rhizostoma 

 (Fig. 612), an animal bearing a close external resemblance to 

 the Medusa. Here the central mouth is entirely absent ; but the 

 tentacula are channelled through their whole length, and their 

 tubes open at their base into the stomach. At the free 

 extremity, the tube of each subdivides and ramifies like the 

 roots of a plant; terminating in a number of small suckers, 

 in the centre of each of which there is a small pore. It has been 

 shown, by placing one of these animals in a coloured fluid, that 

 solid particles, if sufficiently minute, may enter these pores ; but 

 this species must either be nourished by extremely minute 

 animalcules, or by imbibing the juices of other animals, upon 

 which it fixes its suckers. 



1033. According to Ehrenberg, a nervous circle may be 

 detected surrounding the mouth in some animals of this group ; 

 and another, connected with the first, running round the margin 

 of the disk. From the latter he states that filaments may be 

 seen to pass to certain red spots upon the edge of the disk, which 

 he imagines to be eyes. Upon this point, however, there is yet 

 considerable doubt. The animals of this group appear very 

 little sensitive to injurious impressions. They give no signs of 

 feeling the deepest and most extensive wounds of their surface ; 

 and the movements of contraction and dilatation have been seen 

 in parts of the disk almost separated from the rest, as well as in 

 the entire animals when nearly two-thirds of their bulk had been 

 lost by the draining of their fluids. In such instances, the move- 

 ments may be re-excited after they have ceased, by friction and 

 by punctures of the fibrous substance, just like those of the heart 

 and alimentary canal in the higher animals; and they would 

 seem to be of an equally involuntary character. 



1034. Some very curious discoveries have been recently 

 made, in regard to the development of the embryo in the Medusa 

 and its allies. The eggs are transferred, when they leave the 

 ovarial chambers, into certain marsupial sacs or pouches, on the 

 under side of the arms. There they undergo the early stages 

 of their development, and then issue forth in the condition of 



