492 HYDRAFORM POLYPES.- HELIANTHOIDA. 



themselves to the fronds of Sea- Weeds, which are exposed by the 

 reflux of every tide. The deserted shells of Mollusca are the 

 favourite bases of many species ; and a few attach themselves 

 even to these tenants of the deep whilst yet alive. The duration 

 of these structures is various. Many of them do not exist above 

 a year, especially such as are parasitic upon Algae ; but others, 

 particularly those which attach themselves to rocks, probably 

 attain a much greater age. It would seem, however, that the 

 age of the Polypes cannot be measured by that of their cells. 

 In some instances all the Polypes disappear during the winter, 

 dying off like the leaves of a tree ; and they are all renewed 

 with the light and warmth of spring, whilst at the same time 

 fresh branches are produced. In other species, a constant death 

 and regeneration of the Polypes seem to take place. 



1057. One of the most curious phenomena exhibited by the 

 Polypes of this group, is the circulation which may be seen to 

 take place in the stem and branches, and which seems to con- 

 nect the different individuals together. This circulation much 

 resembles that which has been described in the compound Asci- 

 dians ( 979), and is, like it, reversed in its direction at intervals; 

 but only a single current can be seen at a time ; and it is not 

 maintained by any visible movement of the walls of the cavities 

 or tubes in which it takes place. The flow is sometimes very 

 rapid ; it then slackens, and at last stops ; and recommences, 

 sometimes immediately, sometimes after an interval, in the oppo- 

 site direction. Five ebbs and flows have been observed to occupy 

 about fifteen minutes. 



ORDER II. HELIANTHOIDA. 



1058. No very evident links of transition exist between the 

 different subdivisions of the class Polypifera ; and we accordingly 

 pass at once to the next group, of which the common Actinia 

 (Sea- Anemone) may be regarded as the type. Although so 

 different in size and general appearance from the Hydra, this is, 

 like it, a Polype ; and the difference will be found, on close 



