70 THE OPEN BOOK OF NATURE 
bearing a coating of horn or lime. This class in- 
cludes the Brachiopoda, which have double shells, 
one half of which is larger than the other. The 
animal lives in the lower and larger half, and uses the 
upper half, or valve, asa cover. From the creature’s 
mouth two arms extend, and these arms capture 
food. The word Brachiopod is from the Greek 
(brachion=arm, and pous=foot). The Terebratula, 
called the “lamp-shell” on account of its likeness 
to an ancient Roman lamp, is an existent member 
of the class. There are few living brachiopods, but 
in early geological times the species were numer- 
ous, as the rocks testify, and their fossil remains 
form a very important group (see illustrations in 
Fig. 19). 
The Moxuusca (Latin, mollis=soft) are soft- 
bodied animals, generally with no internal skeleton, 
and usually having shells lined with a membrane 
called the “mantle.” The Mollusca are of three 
classes—namely, Lamellibranchia, Gasteropoda, and. 
Cephalopoda. These, I admit, are terrible names, 
but when we know the meaning of them we can see 
how descriptive they are. _ 
1. Lamellibranchia (Latin, lamella=a little plate, 
and branchie = gills)—Oysters, Cockles, Mussels, 
Clams, etc., are members of this class. They are 
called ‘‘ bivalves,’ because their shells consist of 
two valves of the same size. The shells are lined 
with two-lobed mantles, and each animal has a foot 
enabling it to move about. They are gill-breathers. 
Fossil species are found in strata ranging from the 
Cambrian”!Period,” from “which” time they appear 
we 
to have ‘gradually increased in numbers up to 
