48 EVOLUTION OF LIFE. 
form of a spiral. They are represented in the present seas 
by very few genera, Lingula, Terebratula, etc. These Spiral- 
shaped gills were supposed to be used as arms or feet: 
hence their common name of Arm-feet or Brachiopoda. 
The Conchifera are better named Lamellibranchiata, from 
the gills in this class being arranged in the shape of 
plates or lamellæ. This class includes the oysters, clams, 
mussels. The remaining classes of the Mollusca, the Gas- 
teropoda and Cephalopoda, differ from the Acephala not 
only in having heads, but in many other respects. The 
Gasteropoda, or Belly-feet, are so called from these creatures 
moving on that part of their body. This class is repre- 
sented by the Whelks (Fig. 52), most of the shells on the 
sea-shore, and the Snails. The common garden-snail is 
remarkable on account of the great number of teeth which 
arise from the tongue: as many as twenty-five thousand 
are said, according to competent authority, to have been 
discovered. The Cephalopoda (Fig. 54) are distinguished 
by having long arms or feet radiating from the head; hence 
the name of this class, which includes the Cuttle-fish and the 
Pearly Nautilus of the Indian Ocean. Gasteropoda like the 
Dentalium are so rudimentary as regards the development 
of the head, that they may be looked upon as offering the 
transition from the Gasteropoda to the Lamellibranchiata, 
The Spirobranchiata, or Brachiopoda, in their development 
and structure are so closely allied to the Bryozoa that per- 
haps they ought to be considered rather as part of the 
Worms than as belonging to the Mollusca. The curious 
affinities of the Brachiopoda, Bryozoa, and Worms are 
striking proofs for the view that the Mollusca have come 
from the Bryozoa, or animals allied to them, and they from 
the Worms. That the Brachiopoda were the first Mollusca 
that appeared on the earth is at once suggested from the 
enormous number of fossil forms that are found in the 
oldest rocks. They are so numerous that the name “Age 
