MOLLUSCOUS ANIMALS. SI 



Odontophore. The teeth are usually ranged upon it, in a median and 

 two lateral tracts, which have been called the rachis and pleurae terms 

 which are scarcely necessary, and, the first at least, not free from 

 serious objections. The Odontophore is sometimes short but often of 

 great length, its edges behind the mouth being united so as to form a 

 tube, which after passing for a short distance under the oesophagus is 

 rolled or spirally twisted. It seenas that the part in use is soon worn 

 away and that the reserved portion is gradually pushed forward, the 

 tube slitting of^en so as to afford a fresh surface. The form of the 

 teeth both median and lateral, the number in each row and the number 

 of rows vary in different families and different species, and are apparently 

 adapted to the kind of food and the mode of procuring it employed by 

 the animal. Hence, besides the use which may be made of the minuter 

 differences as specific characters, the leading varieties, like the differences 

 in the beaks of birds marking their kind of food or mode of appropriat- 

 ing it, serve to distinguish families, and may now be said to be of great 

 ai^d unquestionable importance. 



Even so late indeed as the publication of Mr. Woodward's valuable 

 manual, the extent to which they could be used seemed very doubtful, 

 and he makes objections to their systematic value being estimated 

 highly. He says : " It must be remembered that the teeth are essentially 

 epithclian cells, and, like other superficial organs, liable to be modified 

 in accordance with the wants and habits of the creatures. The instru- 

 ments with which animals obtain their food are of all others most subject 

 to these adaptive modifications, and can never form the hans of a 

 philosophical system." He adds this note, " the carnivorous opossums 

 have teeth adapted for eating flesh, but are not on that account to be 

 classified with the placental carnivora." 



It may be replied that our object being to bring together creatures 

 of like organization and mode of living, the adaptive modifications of a 

 common plan which determine the kind of food and mode of life are 

 presisely what we ought to make use of, except for the highest divisions, 

 and we find both the teeth of Mammalia, and, as already referred to, 

 the beaks of birds, are of prominent importance in characterising even 

 the great families. We should not allow resemblances or differences of 

 the Odontophore of Mollusks to interfere with the classes or orders 

 which depend on higher characters, nor ought we to use distinctions 

 derived from this one part alone, or we should create an artificial 

 system not perhaps better than others, and more difficult of application 



