OF CENTRAL CANADA PART IV. 



265 



SUB-KINGDOM VII. 



MOLLUSCA. 



The animals of this series comprise a large number of soft-bodied 

 forms in which the blood is essentially colorless and the symmetry 

 bilateral. In most, an external calcareous shell is present ; but in 

 some cases the shell is internal, and in others it is wanting or rudi- 

 mentary. Most mollusks inhabit the sea, but a small number are 

 fresh-water types, and a still smaller number are terrestrial. The 

 marine oyster, whelk, nautilus and cuttle-fish, the fresh-water limnea, 

 and the land snails, are typical examples of existing forms. 



Mollusca are usually classed under two series : Molluscoidea and 

 Mollusca vera. 



A. MOLLUSCOIDEA. 



In the representatives of this series there are no distinct branchiae as 

 in the Mollusca proper, and the heart is either absent or more or less 

 rudimentary. Although connected by these and other organization 

 characters, the two classes into which the Molluscoidea are sub- 

 divided, differ greatly in external configuration. These classes 

 comprise : 1, Bryozoa or Polyzoa, and 2, Brachiopoda. 



I. 



BRYOZOA OR POLYZOA. 



The bryozoons so named from their general moss-like aspect 

 are minute animals of aquatic habitat. They form colonies of indi- 

 viduals which secrete in common a horny, calcareous or gelatinous 

 cellular " exoskeleton " or support, and thus closely resemble Sertu- 

 larians and some other compound Ccelenterates. They possess, how- 

 ever, a distinct oral and anal orifice, with other characters indicating 

 a higher organization. The secreted cell- structure is either leaf-like, 

 plumose, tubular, dendritic, heliciform, circular, or irregular, in 

 form ; and it frequently encrusts other marine bodies. The actual 

 bryozoon may be likened to a minute, digestive sack, with oral and 

 anal orifices, and with a wreath or circle of delicate tentacles at the 

 oral opening. The bryozoa are exceedingly rich in genera and 

 species ; and of late years many fossil forms have been removed from 

 the corals and referred to them, although necessarily on very uncer- 

 tain evidence. 



