MORPHOLOGY OF MOLLUSKS. 221 
the odontophore or lingual ribbon, often absent, is appar- 
ently a modification of the pharyngeal teeth of Annelides. 
Mollusks in general have a heart consisting of a ventricle 
and one or two auricles, and in this respect they are more 
like the Vertebrates than other invertebrated animals; the 
highly developed eye of the squids and their imperfect car- 
tilaginous brain-box are also special characters analogous to 
the eye and brain-box of Vertebrates. Still these features 
are not homologous with the corresponding parts in the 
Vertebrates, and we have already seen that the Tunicata. 
and even the Annelides, are much more closely allied to the 
Vertebrata than are the Mollusks, which should, perhaps, 
be interpolated between the Brachiopods and Tunicates. 
The affinities of the Mollusks are, then, decidedly with the 
worms, rather than with the Vertebrates. 
That the Mollusca are a highly specialized and compara- 
tively modern group is shown by the fact that they began 
to abound after the Brachiopods had had their day in the 
Silurian seas, and had begun to decay and die out as a type; 
the shelled Mollusca supplanted the shelled Vermes or Brachi- 
opods. For the upper Silurian period, and those later, the 
Mollusks prove useful as geological time-marks, especially in 
the Cainozoic period, and so much so that Lyell based his 
divisions of Tertiary time mainly on the shells which abound 
in Tertiary strata. 
Although morphologically the shell of a Mollusk is not 
the most important feature of the animal, it is very charac- 
teristic of them and of great use in distinguishing the species 
of existing, but more especially of fossil, forms; still it is 
liable to great variation, and mollusks of quite different 
families, and even orders, sometimes have shells much alike, 
so that the characters of shells, like many of those drawn 
from the peripheral parts of the body, are liable oftentimes 
to mislead the student. That the Mollusca are a highly 
specialized group is also seen by the enormous number of 
existing species, and their wide geographical and bathymet- 
rical range. There are about 20,000 living and 19,000 
fossil species known, and the group ranks next to the 
winged insects, also a comparatively recent and highly 
