158 



THE -HISTORY OF CREATION. 



bottom of the sea. There now exist but few forms of this 

 class ; for instance, some species of Lingula, Terebratula, and 

 others akin to them, which are but feeble remnants of the 

 great variety of forms which represented the Lamp-shells in 

 earlier periods of the earth's history. In the Silurian period 

 they constituted the principal portion of the whole Mollusc 

 tribe. From the agreement which, in many respects, 

 their early stage of development presents with the Moss 

 animals, it has been concluded that they have developed out 

 of Worms, which were nearly related to this class. Of the 

 two sub-classes of Lamp-sheUs, the Hinge-less (Ecardines) 

 must be looked upon as the lower and more imperfect, the 

 Hinged (Testicardines) as the higher and more fully 

 developed group. 



The anatomical difference between the Lamp-shells and 

 the three other classes of Molluscs is so considerable that the 

 latter may be distinguished from the former by the name of 

 Otocardia. All the Otocardia have a heart with chamber 

 (ventricle) and ante-chamber (aiiricle), whereas Lamp-shells 

 do not possess the ante-chamber. Moreover, the central 

 nervous system is developed only in the former (and not in 

 the latter) in the shape of a complete pharyngeal ring. 

 Eence the four classes of Molluscs may be grouped in the 



following manner : — 



I. Molluscs 

 •without head. 



Acephala. 



II. Molluscs 

 with head. 



Ceplialo2:>hora. 



1. Lamp-shells 

 ( Spirobranchia) . 



2. Mussels 



(Lamellibranchia). 



3. Snails 



(Cochlides). 



4. Cuttles 



(Cephalopoda). 



I. Haploeardia 

 (with simple heart). 



II. Otocardia 

 (with chamber 



aud ante-chamber 

 to the heart). 



i 



