214 APPENDIX. 



individuals in one common life, and grows in a slirub- 

 like form upon any submarine body, such as a shell, 

 a rock, a weed, or even another polypidom to which 

 it is j)arasitically attached. Each polype, in both 

 classes, protrudes from and retreats within its cell by 

 an independent action, and when protruded puts forth 

 a circle of tentacles whose motion round the mouth 

 is the means of securing nourishment. There are, 

 however, peculiarities in the structure of the Poly- 

 zoa which seem to remove them from Zoophytology 

 to a place in the system of nature more nearly con- 

 nected with Molluscan types. Some of them come 

 so near to the compound ascidians that they have 

 been termed, as an order, " Zoophyta ascidioida." 



The simplest form of polype is that of a fleshy 

 bag open at one end, surmounted by a circle of con- 

 tractile threads or fingers called tentacles. The plate 

 shows, on a very minute scale, at figs. 1, 3, and 6, 

 several of these little polypiform bodies protruding 

 from their cells. But the Hydra or Fresh-water 

 Polype has no cell, and is quite unconnected with 

 any root thread, or with other individuals of the same 



