SYSTEMATIC ARRANGEMENT. 



Sub-kingdom MOLLUSCA. 



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

 voracious animals, the Cepltalnpods* , whose ])hysiological 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, kc, belonging to an incalculably remote period of the 

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

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

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

 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 Gasfropodsf 

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

 fication are freely and multifariously exercised. They have a chstinctly 

 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 PterojwdsX, a lunited class of small tAnhght 

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

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

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

 LameUihrmicJdates^, an extensive tribe whose mantle is divided into two 

 lobes, each of which 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 Bracfiiopods\\ have also a shell of two distinct pieces and are destitute 

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

 spiral arms, and they are difterently placed witliin the shell. All Hve attached 

 to foreign bodies. 



* From Ke(f>aKTj (l<cphale) head, and irovs (pons) foot. 

 t From yaa-Ti)p (gastcr) be/li/, and ttovs {i)ous)foot. 

 X From TVT(j)nv (pteron) 'winj, and ttovs {^o\\^) foot. 



§ From lamella, diminutive of lamina, from \dfiva (lamua) a thin plate, and ^pdyxta 

 (bragchia) r/ifls. 



II From /3/ja;(ta)Z' (brachioii) arm, and ttovs {[mis) foot. 



