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It is therefore brought to them in the form of minute animal or 
vegetable organisms, borne straight to their mouth by currents . 
ever flowing into and through the valves of their shell or through 
their protruded syphons; the stream being both generated and 
directed in its course by an exquisitely beautiful arrangement of 
cilia covering their gills. All molluses of this type (Lamell- 
branchiata), and also the Brachiopoda, although possessed of a 
‘simple mouth, guarded by sensitive lips, are not only toothless, 
but absolutely headless. But in the Pteropoda we find a very 
different structure. A Pteropod is an animal passing an active 
life in the open sea, far from shore, swimming by means of a pair 
of fins attached to a head, not well defined, but bearing minute 
tentacles. One of the class, Clio borealis, is rather more than an 
inch in length. On each side of the mouth of this little creature 
are three appendages of a conical form which, when examined with 
the microscope, are found to be covered with minute tubercles, 
and each tubercle proves to be a_ cylinder enclosing twenty prehen- 
sile suckers capable of teing protruded in order to seize any object 
- which may come in contact with the tentacle, and convey it to the 
mouth. The mouth is furnished with a pair of jaws carrying a 
number of spiny teeth. These jaws are contained in two hollow 
muscular cylinders, which, when contracted, force out the jaws on 
either side of the mouth. Inside the mouth is a fleshy organ, the 
upper surface of which is clothed with hooked spines or teeth, 
directed backwards, and placed in regular rows. It is this organ 
_ which is the most constant feature in the dental apparatus of the 
higher mollusca, for it is found under many modifications cf form 
throughout the class Gasteropoda, as well as in the last and highest 
class Cephalopoda. Many species ‘of Gasteropods are destitute of 
eutting jaws, and depend upon the tooth-covered tongue for the 
attrition of their food. 
It is not a tongue in the ordinary sense of the word, and is, 
with more exactness, termed an odontophore or radula. Itisa 
muscular organ, sometimes in the form of a short semi-circular 
ridge, situated between the jaws; in other cases forming the floor 
_ of the mouth and acting in concert with a single upper jaw; 
while, in many of the marine gasteropods, it is of great length, 
often extending in folds backwards to the stomach; or lying 
coiled up at the side of the esophagus. This lingual ribbon usually 
has the edges of its posterior portion rolled together and united so 
as to form a tube, the hinder end of which is closed, but the front 
_ portion opens out as a flat surface in the mouth. It is armed with 
teeth set upon flattened plates, which, in some species, bear single 
teeth, while in others, several teeth are borne by one plate. 
The teeth are arranged always in transverse rows, exhibiting 
a great variety of form in different genera and species. The 
