SYSTEMATIC ARRANGEMENT. 



Sub-kingdom MOLLUSCA. 



At the commencement of the Molluscous series we have a group of active 

 voracious animals, the Cepkalqpods*, whose physiological condition is not 

 much inferior to the Fishes ; and whose history is of especial interest, on 

 account of their being the living representatives of a gigantic fossil race, 

 the Ammonites, &c, belonging to an incalculably remote period of the 

 world's history. They are all marine, inhabitants of deep water, and only 

 a limited number produce a shell of any particular substance. The head 

 and body have the appearance of an oblong bag, the upper portion of which 

 is crowned with a number of arms or tentacles covered with suckers, and, as 

 their name indicates, they walk with the head downwards. We now descend 

 in the scale of organization to an extremely numerous class, the Gastropods f 

 whose habits are of a more sluggish character, and whose powers of calci- 

 fication are freely and multifariously exercised. They have a distinctly 

 formed head and eyes, and, as their name indicates, acquire motion by the 

 dilatation and contraction of an expanded gastric disc. They are marine, 

 lacustral, and terrestrial ; and all are locomotive. The next group in the order 

 of arrangement is that of the Pteropods J, a limited class of small twilight 

 deep-water swimmers, which obtain their powers of locomotion, as their name 

 indicates, by the aid of a pair of wing- like fins ; these only produce a small 

 brittle glass-like shell. The next class in the descending order is that of the 

 Lamellibrancldates\, an extensive tribe whose mantle is divided into two 

 lobes, each of winch produces a separate piece of shell connected together by 

 a horny ligament. None of this group have any head, and a large propor- 

 tion of them are incapable of motion. The last of the Molluscous series, 

 the Brachiopods\\ have also a shell of two distinct pieces and are destitute 

 of any head. Their peculiarity consists in being furnished with a pair of 

 spiral arms, and they are differently placed witliin the shell. All live attached 

 to foreign bodies. 



* From Kf(f)a\r] (kephale) head, and novs (pous) foot. 

 t From ya<rrr)p (gaster) belli/, and novs (pous) /oo^. 

 X From nrepov (ptcron) wing, and irovs (pous) foot. 



§ From lamella, diminutive of lamina, from \dfiua (lamna) a thin plate, and fipdyxt-a 

 (bragchia) gills. 



|| From fipaxlw (brachiou) arm, and novs (pous) fool. 



