5 



bodies are very interesting and are produced by minute 

 vibratile cilia which cover their surfaces and are generally 

 in rapid motion. By means of these cilia the gemmules re- 

 volve on their own axes from spot to spot, sometimes with a 

 rapid gliding motion, at others by sudden springs ; then 

 stopping, and again bounding away to other spots, changing 

 their forms from round to oval or irregular; but having 

 found a situation on which to fix, they generally become oval. 

 The time during which they remain free varies from a few 

 hours to two days, depending on the collateral circumstances 

 of the purity of the water, temperature, and the nature of the 

 spot around which they move. These bodies are frequently 

 called ova; but as they are neither ripened by fecundation 

 nor enclosed in a special membrane, Mr. Hogg proposes to 

 call them reproductive gemmules. After becoming once 

 fixed, they are for ever after incapable of motion, and if they 

 should be removed, rarely, if ever take root again. As soon 

 as they are fixed, there grow from beneath numerous tubu- 

 lous fibres, which serve as roots, by which a firmer hold is 

 obtained ; and in this state they are better enabled to with- 

 stand the violence of the waves. The upper portion of the 

 gemmules gets elongated, the central pulp enlarged, and 

 the first cell is formed. 



Another mode of reproduction is frequently observed in 

 the same genera. When a polypidom has been formed from 

 a gemmule, the tubular fibres by which the polypidom is 

 rooted frequently trail over a considerable extent of surface, 

 and as they do so, get enlarged at short and irregular 

 distances; which enlargements increase in an upward direc- 

 tion, till, first a stem, then a cell and finally a perfect 

 polypidom is formed. This mode of reproduction is easily 

 observed in the Sea-threads ( Laomedta ) so common on 

 our shores throughout the year: the Podded Coralline, 

 (Plumularia cristata,) is another elegant instance of the 

 same thing. 



This order then is capable of reproduction; 1st. By the 

 young pullulating from the parent: 2ndly. By reproductive 

 gemmules enclosed in external vesicles : 3rdly. By shooting 

 from the tubular roots of existing polypidoms; and 4thly, it 

 is sometimes said to occur by division ; as by the artificial 

 division of the fresh water polypes. Some Authors * in- 

 timate that this last manner occurs also in the horny genera, 

 but this I have never observed, and believe never to take 

 place; for I have noted that if a branch be either cut 

 off, or thrown off spontaneously, the utmost it appears to 

 be capable of doing is to survive for a very short time. 



* Jones' Animal Kingdom, p. 47. Sect. 0(3. 



