MOLLUSCOUS ANIMALS. 323 



ing the highest position in the animal kingdom, since both the average 

 elevation of its membprs is far above that of the other sub-kingdoms, 

 and its type admits, when most fully developed, of an incomparably 

 higher condition than any of the others. It is equally certain that 

 Protozoa must have assigned to theni the lowest position as represent- 

 ing only the rudimentary or embryonic condition of all other animals. 

 Hence the unwillingness manifested by some to admit this sub- 

 kingdom, the supposition being adopted that all its members may be 

 only degraded forms of some of the others. Since, however, many of 

 them are known to pass through a definite series "of changes con- 

 stituting a life history, whilst retaining all through the characteristics 

 of the supposed sub-kingdom, and displaying no trace of the peculiar 

 type of any of the higher ones ; since, even in their great simplicity 

 of structure they can be thrown into well-distinguished classes ; and 

 since analogy with what is found in each of the other sub-kingdoms 

 would create an expectation of meeting with an Embryonic type, as 

 well as those representing different tendencies of developement, the 

 balance of probability is, on the whole, greatly in favour of the fifth 

 sub-kingdom. The proper order of the remaining three great branches 

 of the animal kingdom is not very difficult of determination, although 

 some writers of very high authority may have fallen into error respect- 

 ing it. We often find Mollusca placed next to Yertebrata, and we find it 

 not unfrequently asserted that Articulata and Mollusca stand in equal 

 relation to the higher branch and should be placed one on each side be- 

 low it. Without doubt the cartilaginous brain case of the highest class of 

 Mollusks suggests a comparison with Vertebrates, sufficient at least to 

 afford another proof of these being the highest in the Molluscan series, 

 but if we inquire concerning the general characteristics of the sub- 

 kingdoms, we must perceive that the segmented structure of Articulata, 

 their general higher development of the nervous system, and the pre- 

 vailing predominance in them of muscular energy raise them above 

 Mollusca, which are generally sluggish, and in which the nutritive 

 system is evidently the most developed. We may then assume the 

 true order of the greatest divisions of the animal kingdom to be, 1st, 

 Vertebrata ; 2nd, Articulata ; 3rd, Mollusca ; 4th, Radiata ; 5th, 

 Protozoa. To the first belong the highest power — the greatest deve- 

 lopment of the brain and of the organs of sense ; to the second, the 

 greatest degree of muscular development and its result activity. To 

 the third, peculiar development of the nutritive system, with diminish- 



