imn 
UC ch. 
ZOOLOGY. 49 
of Mollusca” has been given to the time during which 
these creatures lived, there being found in great numbers 
also representatives of the other classes of Mollusca, 
though not in so great a profusion as Brachiopods. The 
Brachiopoda of the present seas are restricted to a few 
genera, the class having nearly died out. This is true, com- 
paratively speaking, of the rest of the Mollusca, though 
not in so great degree. This fact of the abundance of the 
Mollusca in the oldest rocks, and of the Brachiopoda in 
particular, harmonizes with the facts of their structure and 
development in showing that the group must have branched 
off from the main trunk (worms) very early in time, and that 
of the Mollusca the Brachiopoda are the oldest. In some 
Gasteropoda the feet appear modified as wings, as in Hyalza 
and Cleodora, constituting the Pteropoda, and offering the 
transition to the Cephalopoda. This view is confirmed by 
the embryos of the Gasteropoda, Pteropoda, and Cepha- 
lopoda (Fig. 53) being so very similar. The Gasteropoda 
breathe both by gills and lungs; examples of the gill-breath- 
ing kind are seen in the Whelks (Buccinum) (Fig. 52) often 
picked up on the sea-shore while the garden-snail will repre- 
sent the lung-breathing kind. The beautiful Carinaria, with 
its delicate propeller, is a highly specialized gill-breather. 
The Cephalopoda are the most highly organized of the 
Mollusca. The Cuttle-fishes (Fig. 54), with their long arms, 
are familiar to all who have read the “ Toilers of the Sea.” 
In them we find the nervous system well developed, eyes 
are present, the viscera are large, while blood circulates 
through arteries and veins. The Cuttle-fish is able to conceal 
itself through emitting a very brownish-black fluid, which 
is contained in the so-called ink-bag. They breathe by two 
gills, but in one genus, the Pearly Nautilus, four gills are 
present. The Nautilus is the only living representative of 
myriads of fossil forms, the Ammonites, and is probably the 
ancestor of the two-gilled اص‎ The evidences of 
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