POLYPI. 



14 J) 



for the most part attached to some shell or base, 

 which may be either of a horny or calcareous na- 

 ture. The form of this shell admits of almost infi- 

 nite variety. In some it constitutes the external 

 surface of the animal, and encloses the flesh in a 

 general sheath, leaving only openings at the extre- 

 mities of the tubes for the expansion of each set 

 of tentacula surrounding the respective mouths. 

 Sometimes these tubes are joined together endwise, 

 like the branches of a tree, leaving lateral apertures 

 for the protrusion of the tentacula of each separate 



polype: this the case in the Sertu- 

 iaria. (Fig. 60.) At other times 

 the tubes are placed parallel to 

 each other, like the pipes of an 

 organ, with transverse partitions 

 at regular intervals : such is the 

 structure of the Tubipoia musica^ 

 as shown in Fig. 61. In Fig. 62, a 

 portion of the tubes is seen highly 

 magnified, and laid open, to show the polypes in 

 their interior. 



In some species the horny base is fashioned into 

 a number of cells, each of which serves for the pro- 

 tection of its respective polype. These cells are 

 generally placed at the extremity of the branches, 



