516 REMARKS ON THE CLASSIFICATION OF MAMMALIA. 
limited, Rhizopoda and Porifera. The latter class, consisting of the 
sponges, many naturalists assume to be vegetable. Agassiz contends 
that Rhizopoda probably are so also, and argues from several known 
cases, some of them ascertained by his own observations, that the 
remaining Infusoria are embryonic forms of various worms. I must 
say that I very obstinately cling to the conviction of the animal » 
nature of sponges. IJ can see no pretence for maintaining the vege- 
table nature of Rhizopods, and I| believe that when all embryonic 
forms of other creatures, as well as all true vegetables, are with- 
drawn, the class Infusioria will still be sufficiently numerous; as, 
therefore, these groups cannot naturally be included in Radiata, and 
in the absence of a definite nervous system and of the distinct organs 
for the different functions are sufficiently distinguished from. all 
others, I think Protozoa may safely be added as a fifth branch or 
sub-kingdom of the animal kingdom. Most naturalists now admit 
Amphibia as a fifth class of Vertebrata, and thus in these important 
cases the numbers are brought to correspond. Perhaps a reasonable 
combination and limitation of classes in Articulata and Mollusca 
might produce the same effect, and if we consider the position of 
Radiata and Protozoa we may see no cause to wonder at their pre- 
senting a reduced number of classes omitting the modifications of 
lower development found in the other divisions. 
I have endeavoured to express by a diagram my idea so far as I 
have yet carried it out.* Several particulars, perhaps, claim fuller 
explanation and defence than I have given them, but I hope enough 
has been done to enable those who are interested in the subject to 
judge of the advantages arising from my plan. In these few re- 
marks I have been chiefly anxious to show the completeness of the 
analogy as to all the leading divisions between mammalia and birds, 
but I wish the whole to be considered rather as suggestions thrown 
out for examination than as a carefully elaborated system, which I am 
prepared to maintain in every particular. 
* It is not thought necessary to engrave this diagram, the nature of which will be under- 
stood from the preceding observations. In the long interval between the reading and the 
publication of this paper, in consequence of its having been mislaid, the author has given 
much attention to Owen’s classification of mammalia, which he has studied with great plea- 
sure and profit though without being inauced to abandon the leading features of his own 
scheme, 
