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 Molluse 
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-shells, 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 (auricle), 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- 
Hence the four classes of Molluscs may be grouped in the 
following manner :— 
1. Lamp-shells I. Haplocardia 
I. Molluscs (Spirobranchia). (with simple heart). 
without head. 2. Mussels i 
Acephala. (Lamellibranchia). II. Otocardia 
3. Snails (with chamber 
parents (Cochlides). and ante-chamber 
ashe si x 4. Cuttles to the heart). 
ritlaee iteer (Cephalopoda). 
