CLASSIFICATION OF THE ANIMAL KINGDOM 



30 



diversity and the differences by pretending 

 that they can be included in one phylum. 

 Besides the Enteropneusta and the Ptero- 

 branchia, there have been placed here also 

 the Planctosphaeroidea, the Phoronida, 

 and the Graptozoa. The latter two have al- 

 ready been accepted as distinct phyla in 

 a previous part of this classification (fol- 

 lowing many other classifications). The 

 Planctosphaeroidea, consisting of certain 

 ciliated larvae of unknown affinities, can- 

 not be associated with any known adults. 

 It is therefore impossible to combine them 

 confidently with any phylum. It seems 

 necessary to maintain them as a distinct 

 group at the present time. 



Chordata. It is now more than seventy- 

 five years since the vertebrates and some 

 supoosed relatives were first combined un- 

 der this name. Nearly all subsequent clas- 

 sifications have accepted this arrangement, 

 although the included groups vary some- 

 what. At the extreme the Chordata have 

 included the Hemichordata, Tunicata, and 

 Cephalochordata, as well as the Verte- 

 brata. 



The inclusion of the Hemichordata is 

 unequivocally rejected by Hyman (1959) 

 and others. The arguments seem to be well 

 founded, involving the absence of any sub- 

 stantial similarity in major features. 



The Tunicata, included in the Chor- 

 data as a matter of course in many works, 

 are excluded here because the similarities 

 appear to be far outweighed by the differ- 

 ences between tunicates and vertebrates. 

 Even if an homologous notochord be pres- 

 ent, even if perforations of the tracheal 



walls do occur, the extreme differences in 

 arrangement of the digestive tract, the ab- 

 sence of paired structures in tunicates, 

 the presence of the tunic and the substance 

 tunicin in tunicates, the reversible blood 

 flow in tunicates, and so on, all seem to 

 deny a similarity so close as to justify in- 

 clusion in one phylum. With the tunicates 

 included, the Chordata are extremely dif- 

 ficult to define effectively, except on the 

 two features of notochord and pharyn- 

 gotremy. With the tunicates excluded, the 

 list of features held in common by the re- 

 maining groups is considerably increased. 



The Cephalochordata can much more 

 reasonably be united with the vertebrates. 

 Nevertheless, the oral hood, the atrial sys- 

 tem, the brown funnel, the multiple paired 

 solenocytic nephridia, the single-layered 

 epidermis, the peculiar liver-pouch, the ab- 

 sence of a heart, the multiple ductless gon- 

 ads, and other features seem to show that 

 the group is "more widely separated from 

 the lowest fish than the lowest fish from a 

 bird or mammal" (Parker & Haswell, 

 1897). It seems appropriate to recognize 

 these substantial differences at the phylum 

 level. 



The use of the name Chordata for the 

 restricted concept seems to be unnecessary 

 as well as inappropriate. The name is 

 younger and less well known than Verte- 

 brata. It would seem to be an unnecessary 

 name, based on overemphasis on a very 

 few features held in common by the groups 

 involved. At the most it might be consid- 

 ered to be a sort of "superphylum," but 

 even thus it could reasonably include only 

 the Cephalochordata and the Vertebrata. 



Sources of class and order arrangements 



Protozoa. The five classes of Protozoa are 

 almost universally agreed upon in recent 

 works. The use of the subphyla is not so 

 widespread. Protociliata may be consid- 

 ered to belong in the Plasmodroma rather 

 than in the Ciliophora, being there treated 

 as a separate class. Suctoria are sometimes 

 united with Ciliata as a subclass. Telo- 

 sporidia (Amoebosporidia) are sometimes 

 separated from Sporozoa as a third sub- 

 phylum. 



This is the classification of Kudo 

 (1954) except for: 1] the elevation of 

 Haplosporidia and Sarcosporidia to sub- 

 class level, following Hyman (1940); and 

 2] a few cases of different choice from 

 available synonyms. 



Porifera. Three classes are recognized in 



most recent works, including the Treatise 

 of Invertebrate Paleontology (E, 1955), 

 but most paleontology books list also the 

 Receptaculitida as incertae sedis. Inasmuch 

 as it cannot be justified in any of the three 

 classes, it must stand as a separate class. 

 (The Nidulitida are now thought to be 

 algae rather than sponges, and the Pleo- 

 spongea are treated as a separate phylum). 

 Many works divide each of the three 

 classes into subclasses. In some cases these 

 subclasses are based on features now be- 

 lieved to be of minor importance or taxo- 

 nomic value, and the view of de Laubenfels 

 is here adopted that subclasses do not add 

 effectively to the classification of this 

 group. Aside from this, the arrangement 

 here adopted is substantially that of both 

 Hyman and the Treatise (E). 



