194 NATURAL HISTORY OF 



referred to, exhausted tlie knowledge on the subject 

 so far as the English species are concerned. In the 

 Penned ulida the zoophytes are more or less pen-shaped, 

 having a stalk which is often buried in the mud, sur- 

 mounted by a rachis, upon which, either directly or on 

 leaves, are borne the polyps. The stem is made up of 

 horny and calcareous matter, and is often quadrangular 

 in section, and sui^mounted throughout by the investing 

 coenenchyma. The stalk and rachis are traversed by 

 longitudinal canals which are connected with the canals 

 of the coenenchyma. Amongst all the PennatuUda 

 there are not only mature polyps, but immature or 

 rudimentary ones called zooids, the peculiarities of 

 which have been stated. The leading differences 

 between the three families of this type are stated in 

 the table already given, and relate to the arrangement 

 of the polyps. 



