56 POPULAR HISTOEY OP THE AQUARIUM. 



ever, commences in a spinous projection from the lower 

 rim, and this afterwards expands into a cell to which 

 otliers are added. The polype in each cell is protected bj 

 an elastic membrane, which retracts into its very depths 

 when the polype is projected, but, when that is withdrawn, 

 forms a projecting rim, or lappet, beyond the edge of the 

 cell. The polype is protruded in through an opening in 

 this membrane, in three circular slides, hke the joints of 

 a telescope. The top, or head-joint, is crowned by twelve 

 ciliated arms ; the second, edged by a scalloped frill, and 

 the third, or basal joint, has a projecting point. The whole 

 is most exquisitely formed, and of a most delicate, filmy 

 transparency. 



To observe the course of the digestive system in very 

 minute animal frames is very difficult, requiring the adjust- 

 jnent of a high power, to the medium in which, to produce 

 the continued actions of life, the object must be placed. 

 In order to assist in the observations, a colour has been 

 introduced into the element, which, being readily imbibed 

 by the polype, is seen in circulation through the body. 



There is a continued motion of cilia going on, whicli 

 enables the polype to bring a current of nutritious sub- 

 stance within its power, as well as to throw off any dis- 



