110 THE INVERTEBRATE PHYLA 



(mesoderm) of the ciliates; it is not therefore a new formation. 

 Various cavities such as the nephridial cavity, blood cavity, 

 secretory cavities, lymph cavities, rhynchocoel and pericardium 

 are all regarded as being part of the coleomic system. There are 

 primary cavities, those without an epithelial lining, and secondary 

 cavities, those with an epithelial lining. The perigastrocoel is a 

 space lying alongside the gut and it too has become lined with 

 epithelium. Those animals that have such a perigastrocoel and 

 which are also unsegmented Hadzi places in his first metazoan 

 phylum, the Phylum Ameria. Included in the Ameria are the 

 following groups: Platyhelminthia, Coelenterata, Gastrotricha, 

 Rotatoria, Kinorhyncha, Mollusca, Priapuloidea, Acanthocephala, 

 Nematoda, Nematomorpha, Nemertini and Kamptozoa. 



The second phylum is the Phylum Polymeria. These animals 

 are all segmented and the perigastrocoel becomes initially broken 

 up to form a series of cavities. In some of the higher Polymeria 

 the cavities become reduced, as in the Hirudinea. Included in 

 the Polymeria are the following groups : Annelida, Sipunculoidea, 

 Echiuroidea, Crustacea, Pantapoda, Xiphosura, Arachnida, Chilo- 

 poda, Diplopoda and Insecta. 



The third phylum is the Phylum Oligomeria. These animals 

 have at some stage of their development adopted a sessile habit 

 and this has led to a reduction in body segmentation. In the 

 Oligomeria are placed the Phoronidea, Brachiopoda, Bryozoa, 

 Chaetognatha, Echinodermata, Pogonophora, Enteropneusta and 

 Pterobranchiata. 



The fourth phylum is the Phylum Chordonia. In this are placed 

 the Vertebrata and the Tunicata. 



Hadzi does not consider that the higher invertebrates can be 

 satisfactorily classified into Protostomia and Deuterostomia. 

 Instead he thinks that the line that gave rise to the higher 

 arthropods also gave rise to the echinoderms. 



Marcus's classification and that of Grobben have more in 

 common than either has to Hadzi's, the greatest difference being 

 in the position of the platyhelminthes ; Grobben thinks they are 

 primitive, Marcus thinks they are advanced. The major difficulty 

 in their classification lies in the placing of phyla other than the 

 Annelida, Mollusca, Echinodermata and Protochordata. All the 

 remaining small phyla are difficult to place. It is possible that some 



