6 CLASSES OF THE MALACOZOA. 



usually applied to these mollusks and refers to the breathing 

 organs, consisting of a pair of flat, membranous gills attached to 

 the mantle. Pelecypoda is a preferable designation for the sake 

 of uniformity with the class names in the Encephala and inas- 

 much as, in the cephalopods and gastropods, the next subsidiary 

 division into Orders is distinguished by names indicating difl^'er- 

 ences in the breathing organ s. Both the breathing and locomotive 

 organs (the latter rudimentary) restrict this class to an aqueous 

 life : they are marine and fliiviatile in distribution. The shell is 

 generally in two pieces symmetrically applied to the sides of the 

 animal, but there are, in one of the orders, small accessory pieces, 

 plates or valves applied upon or on the sides of the hinge line. 

 Anciently shells were grouped into univalves, bivalves and 

 multivalves, and the latter included the chitons (gastropods) the 

 pholades (pelecypods), as well as certain lepades (crustaceans), 

 etc. 



The Molluscoida include the Brachiopoda, Tunicata and 

 Bryozoa. The reciprocal relations of these groups scarcely 

 justify their union as classes under this head, whilst on the other 

 hand their individual characteristics are not sufficiently important 

 to give them each the same systematic value as mollusca. 



Glass I. Brachiopoda are covered with a bivalve shell, the 

 valves of which are placed dorsally and ventrallj?" instead of being 

 applied to the sides of the animal, as in Pelecypoda. The name 

 is derived from two long ciliated organs developed from the 

 sides of the mouth, which were originally termed arms — whence 

 the term arm-footed ; they are, however, breathing organs, and 

 Brachionobranchia has been proposed as a more correct desig- 

 nation, but will scarcely obtain the preference over the shorter 

 name so long in use. 



Glass II. Tunicata. Instead of a shell the tunicates are pro- 

 tected by an elastic integument, formed of cellulose, and having 

 two orifices — mouth and anus. An inner sack with numerous 

 openings forms the breathing organ, and the nervous system 

 consists of a single ganglion placed near the mouth. 



Glass III. Bryozoa or Folyzoa, are microscopic animals, each 

 protected by a calcareous or chitinous shell : they live in colonies, 

 attached, and their aggregated shells form coralline incrustations 

 upon the surface of molluscous shells or other marine or fluviatile 

 objects. 



Only one of the classes of Molluscoida will be included in this 

 work. The brachiopods have so long been considered true 

 mollusca, their shells are so similar to those of the Pelecypoda, 

 they occur together, and have so occurred from an early geo- 

 logical period, so that practically they continue to be as much 



