27 



pletely nebulous that it no longer serves 

 any clear purpose. Use of the subkingdom 

 and series names herein is simply a recog- 

 nition of their use in many recent classifi- 

 cations. 



Division of the Histozoa (Metazoa) 

 into grades, series, or divisions has been at- 

 tempted many times. Most of the groups so 

 adopted in the past are employed in the 

 face of obvious defects in the form of ex- 

 ceptions. Not all Radiata show any form 

 of radial arrangement, and not all radially 

 arranged animals are put in the Radiata. 

 Bilateria included animals with quite di- 

 verse body plans, some with virtually no 

 paired structures, no obvious "sides," no 

 anterior and posterior, and only a remote 

 similarity to the obviously bilateral ani- 

 mals. Some groups placed in the Schizo- 

 coela form their coelom in the enterocoe- 

 lous manner, and at least one group placed 

 in the Enterocoela forms its coelom by the 

 schizocoelous method. Articulata has in- 

 cluded animals that are not segmented. 

 And so on. 



It is here concluded that the histozoan 

 (or eumetazoan) phyla cannot be grouped 

 readily into clear-cut series. The number 

 of these phyla is not so great as to force 

 subdivision of the subkingdom, but custom 

 seems to be sufficient justification for indi- 

 cating some grouping of them. At this 



Notes on Subkingdoms and Phyla 



point it appears that the most useful group- 

 ing is the one based on the type of body 

 cavity. Accordingly four groups are here 

 recognized, those phyla with an enterocoel 

 or gastrovascular cavity only, those with 

 no cavities except a digestive tract, those 

 with a pseudocoel, and those with a coe- 

 lom. These groupings are all well known 

 from Hyman (1940) and other works. 



No satisfactory name exists for the 

 first of these four groups. Radiata is inap- 

 plicable to many forms. Protaxonia is 

 based on a concept of embryonic axes 

 which would include extraneous groups. 

 Enterozoa and Enterocoela were originally 

 applied to much larger concepts. On the 

 whole, Enterocoela is the most appropriate 

 in meaning, and it is adopted here. 



Radiata and Bilateria. The Histozoa or 

 Metazoa have sometimes been divided into 

 the Radiata (Coelenterata and Cteno- 

 phora) and the Bilateria (all others) on 

 the basis of their general body arrange- 

 ment. The distinction is here held to be a 

 fictitious one, because Ctenophora are 

 much less radial than some Echinoder- 

 mata, and such an animal as a bryozoan is 

 so completely different in body arrange- 

 ment from an annelid worm that it is 

 meaningless to say that they are both bi- 

 lateral. 



Phyla and rejected 



Cyathospongia. Recent works on this ex- 

 tinct phylum have adopted either Pleo- 

 spongia or Archaeocyatha as the phylum 

 name. Both of these names are of later 

 date (1937) than Cyathospongia (1935), 

 and Archaeocyatha has been used more 

 consistently for one of the included classes. 

 There seems to be no firmly established 

 usage that prevents us adopting the oldest 

 name. 



Mesozoa. There appears to be no reason 

 for not adopting the name now in wide use 

 for this phylum, especially as it is the old- 

 est name (1877). The name has also been 

 used at the subkingdom level, where the 

 synonym Agnotozoa seems to be more ap- 

 propriate. 



Monoblastozoa. A new phylum named 

 here for the unique metazoan Salinella, 

 which has too long been left excluded 

 from the classifications of animals. It con- 

 tains only one genus and one species. The 

 animal consists of a single layer of similar 

 cells surrounding an internal tubular tract 

 which has a "mouth" at one end, an "anus" 



phylum groups 



at the other. The cells are thus simultane- 

 ously both "ectoderm" and "endoderm"; 

 they are ciliated on both surfaces. Repro- 

 duction is asexual (by transverse fission), 

 and there are indications of a sexual proc- 

 ess in the form of fusion of two individ- 

 uals. It is possible that a ciliated unicellu- 

 lar larva results from the sexual process. 



Hyman suggested (1940) the phylum 

 status for this peculiar animal, but she did 

 not propose such a phylum directly and 

 left Salinella unassigned to any group. 



Graptozoa. The graptolites have had a 

 more varied history than most other major 

 groups. They were for years assigned to the 

 Hydrozoa in the Coelenterata, but have 

 more recently been transferred to the 

 Hemichordata. In 1959 Hyman examined 

 the arguments supporting the hemichor- 

 date assignment. After effectively dispos- 

 ing of all of these, she left the group 

 without clear assignment, although she pre- 

 sumably retained them in the Hydrozoa, as 

 she had doubtfully done in 1940. 



Some features of the skeleton of grap- 

 tolites are not duplicated in the Hydrozoa, 



