Notes on the Taxa 



Subkingdoms and 



Animalia. Division of the Animal King- 

 dom into four subkingdoms is seen in 

 many recent classifications, although some 

 writers prefer to list the Parazoa, Mesozoa, 

 and Eumetazoa as branches of Metazoa in 

 contrast to the Protozoa. The use of the 

 additional level "branch" is difficult to jus- 

 tify where so few groups are involved, un- 

 less there is definite information on the 

 phylogeny of these groups. Such knowl- 

 edge of the relationships of the groups is 

 lacking, or, at best, highly speculative. The 

 groups are generally recognized at the 

 phylum and subkingdom levels, and these 

 seem to be adequate for classification ex- 

 cept within the Metazoa proper (see be- 

 low). 



Inasmuch as it is often stated that ani- 

 mals are either one-celled or many-celled, 

 it would seem to be necessary to accept the 

 older subdivision of Animalia into two sub- 

 kingdoms, Protozoa and Metazoa. The fact 

 is, however, that many undoubted proto- 

 zoans exist only in aggregations of many 

 cells, often with as much division of labor 

 between cell types as in some undoubted 

 metazoans. Removal of the Protozoa to a 

 separate kingdom Protista solves part of 

 this problem, but the remaining animals 

 still represent the three very different basic 

 structures: 1] a vase-like cylinder open 

 at one end and with several types of cells 

 in the walls but with the internal cavity 

 not serving for digestion, 2] a solid body 

 consisting of one layer of cells around a 

 central cell or group of cells, and 3] a 

 multicellular body with internal cavities of 

 which one is usually a digestive tract and 

 with walls of one, two, or three layers of 

 cells. 



These three types of construction are 

 so different as to require recognition as pri- 

 mary divisions of the kingdom. With the 

 Protozoa (when these are treated as ani- 

 mals), they form the four subkingdoms 

 employed here: Eozoa, Parazoa, Agnoto- 

 zoa, and Histozoa. 



their subdivisions 



Eozoa and Agnotozoa. In the choice of 

 names for subkingdoms, it has been felt 

 that only slight advantage results from 

 having a single phylum known by a differ- 

 ent name than is used for its subkingdom. 

 However, with only four subkingdoms in- 

 volved and with two of these consisting of 

 two or more phyla, it appears to be rea- 

 sonable to be uniform in this respect and 

 use separate names for the subkingdom 

 and phylum that include the protozoans 

 and for the subkingdom and phylum that 

 include the mesozoans. 



Parazoa. The Porifera have long been rec- 

 ognized as constituting a group distinct 

 from the rest of the many-celled animals. 

 The extinct Cyathospongia, under one of 

 the three available names, were placed 

 with the sponges by Okulitch and others, 

 and as a separate phylum in the Parazoa 

 by Pearse and others. 



Histozoa. This name is accepted here be- 

 cause of the great ambiguity of the more 

 familiar name Metazoa. As explained 

 above, the incorporation of many cells into 

 one body is not distinctive of any major 

 group of animals, even if single-celled 

 adult structure is found exclusively in one 

 group. It was the desire to retain Metazoa 

 which has led many writers to list the 

 Parazoa, Mesozoa, and Eumetazoa as 

 branches of a subkingdom Metazoa. Inas- 

 much as Metazoa cannot be effectively de- 

 fined, to the exclusion of all Protozoa, it 

 seems to be more realistic to recognize 

 three or four subkingdoms of animals on 

 the basis of the general body construction. 

 Attempts to divide it on the basis of cell 

 number are arbitrary and misleading. 



The use of any of these subkingdom 

 groups is of questionable value; it is the 

 phyla that are important and that are most 

 often definable. The distinction between 

 even Protozoa and Metazoa is so com- 



