230 PALEONTOLOGY. 



variety of forms. Sometimes it is internal, and appears 

 like a rudimentary bone. The mollusca are for the most 

 part marine animals. Many are lacustrine, and a few are 

 terrestrial. 



The character of the shells varies with the habitat of the 

 species. They are for the most part large and heavy in the 

 marine, and light and delicate in the lacustrine and terrestrial 

 species, although there are many exceptions to this rule. 



The primary division of this class depends on the deve- 

 lopment of the nervous system, and on the presence or 

 absence of the ganglia, that represent the brain. The first 

 form the ENCEPHALOUS, the second the ACEPHALOUS section. 



The ACEPHALOUS Molluscae are all aquatic. Their dis- 

 tribution into classes depends on the character of the integu- 

 ment and the shell, and on the modifications of the respiratory 

 organs ; it consists of three classes : 



The Tunicata have no shell, and are enclosed in an elastic 

 muscular sac, with two openings ; some are solitary, others 

 are social, and organically united in groups like polyps. 



The Brachiopoda are enclosed in a bivalve shell. They have 

 two long spiral arms developed from the sides of the mouth, 

 and fixed to an internal frame work. They respire by 

 means of the vascular membrane which lines the shell. 



The Conchifera have a bivalve shell ; they respire by 

 laminated branchiae, attached to, but distinct from, the 

 mantle ; most of them have a fleshy foot for locomotion. 



The ENCEPHALOUS Molluscae are divided into classes ac- 

 cording to the modifications of the organs of locomotion. 



The Gasteropoda are, for the most part, enclosed in a 

 univalve shell. They creep by means of an undivided 

 muscular disc or foot, situated on the under surface of 

 the body. 



The Pteropoda swim by two membranous wing-like ex- 

 pansions situated at the sides of the neck. When they have 

 a shell it is thin, fragile, and univalve. 



The Cephalopoda have all or part of their locomotive 

 organs arranged round the head, in the form of eight or 

 more arms or tentacula, with or without sucking discs. 



Some have internal symmetrical bones, as the Sepia and 

 Loligo. Others have an external, many-chambered shell, 

 with a siphuncular tube passing through the chambers, like 



