12 DEVELOPMENT OF 



ferent degrees of development. The border of the dis- 

 coid or campanulate body of the Medusa is also frequently 

 furnished with extensible filaments or arms, of greater or 

 less length, which either depend loosely beneath the 

 animal, or twine themselves about from side to side in an 

 undulatory manner, and serve most probably for the pre- 

 hension of food (these are the so-called marginal tentacula 

 or filaments). In many species, however, these organs 

 are wanting. Similar filamentary appendages and fringes, 

 in several species, surround the mouth, and stomach, 

 which are placed in the middle of the bell- shaped, or 

 depend from the centre of the discoid body, and they 

 occur also in several situations on the under surface 

 of the animal. 



Although the Medusa are usually observed to float 

 with the currents of water, still they are possessed of vo- 

 luntary motion, and perform a sort of rowing movement, 

 peculiar to certain forms among the lower animals ; this 

 is effected by a powerful contraction of the body, by 

 which the water contained in its hollow is expelled; and 

 by such alternate contraction and dilatation of the body 

 they are enabled to propel themselves with tolerable 

 rapidity. 



With regard to the propagation of the Medusa, it had 

 been observed, that the larger and more common species 

 appear on our coasts and out at sea, in vast numbers at 

 certain seasons of the year, and also that in the spring 

 and beginning of summer the number of smaller or 

 younger individuals preponderated. From the latter 

 phenomenon, it might be concluded that the propagation 

 of these animals took place probably towards or in the 

 winter ; still (when we consider the fact of their complete 

 disappearance at that season) it could not occur in the 

 same localities in which they swarm in such great num- 

 bers during the warmer months. Moreover, it was not 

 known whether their propagation proceeded at the bottom 

 of the sea or towards the surface, or whether the young, 

 after birth, underwent a metamorphosis or not. 



