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MOLLUSCA. 



Order I. MOLLUSCA CEPHALA.—Head distinct from 

 the body, bearing the lips or jaws. 



' Sect. I. NATANTIA. — Organs of progressive motion Jilted 

 for swimming. 



( CEPHALOPODA. — Fins in the form of tentacula, surrounding the 

 ( mouth. Marine. 



j Nautilid^ — With a multilocular shell. 

 ( Sepiad^e. — Destitute of a multilocular shell. 



. Sect. II. GASTEROPODA. — Organs of progressive motion 

 fitted fur creeping. 



I. PULMONIFERA. — Respiring in air by means of a single 

 pulmonary cavity. 



Terricola. — Resident on land. 



Aquatica — Resident in the water. 



In. BRANCHIFERA Respiring in water. 



Nudibranchia — Branchiae external, pedunculated, and plu- 

 mose. 



Pectinibranchia. — Branchiae in the form of sessile, pecti- 

 nated ridges, contained in a cavitj^ 



Order IL MOLLUSCA ACEPHALA.— Destitute of a 



distinct head or neck. 



Sect. I. CONCHIFERA.— Co«cn«g testaceous. 



I. BRACHIOPODA — Mouth with a spiral arm on each side, 

 fringed with filaments. 

 Pedunculata. Shell supported by a cartilaginous stalk. 

 Sessilia. One valve of the shell cemented, fixed. 

 . II. BIVALVIA — Mouth destitute of spiral fringed arms. 

 AsiPHONiDA — Cloak open, without syphons. 

 SiPHONiDA — Cloak more or less closed, forming syphons. 



Sect. II. TUNICATA— Coi;eri«^ soft. 



DicHiTONiDA. — Inner tunic detached from the external one, 



and united only at the two orifices. 

 MoNocHiTONiDA. — Inner tunic adhering throughout to the 



external one. 



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