108 THE INVERTEBRATE PHYLA 



genitalia are more simple. Their embryology indicates a possible 

 relationship with the polyclads. 



The Platyhelminthia have most of their organ systems reduced 

 and simplified; the animals are not therefore primitive. The 

 coelom is reduced to the ciliated ducts of the reproductive organs. 

 The Acoela are not the most primitive of the Turbellaria (Fig. 37). 



The Aschelminthia include the Nematoda, Rotatoria, Gastro- 

 tricha, Nematomorpha, Kinorhyncha and Priapulida, some of 

 which show a spiral pattern of cleavage. It is not clear if the 

 Aschelminthia are a closely related group of animals. 



The Mollusca and Annelida are derived from a common 

 ancestry. The ventral pharyngeal sac of the archiannelids is 

 similar to the radula sac of the molluscs and the teeth of the 

 Eunicidae show plates that are similar to the radula teeth. The 

 primitive molluscs such as Neopilina may be segmented. 



The Articulata (Arthropoda) arose several times from the annelid 

 stock. The Pentastomida, Onychophora and Tardigrada are three 

 groups that are quite distinct from one another, though similarities 

 between the legs, body cavity and gonads can be used to form a 

 link between the Tardigrada and the Onychophora. The Trilobita 

 gave rise to the Arachnomorpha. The crustacean resemblances 

 of the trilobites are due to homoiology; the independent deriva- 

 tion of similar structures in separate lines that are phylogenetically 

 related. (Other examples of homoiologous organs are compound 

 eyes, trachea and malpighian tubules.) The basic line that gave 

 rise to the Crustacea also gave rise to the Antennata from which 

 came the Myriapoda and the Insecta. 



The Deuterostomia are a smaller and more compact group than 

 the Protostomia. The hemichordates contain the Enteropneusta 

 and the Pterobranchiata. The Enchinodermata and the Enter- 

 opneusta are linked together by the dipleurula larva. The 

 ancestor of the Hemichordata then being postulated as giving rise 

 to the Tunicata and the Vertebrata. 



Hadzi Classification 



The relationship that Hadzi (1944, 1957) postulates between 

 the various invertebrate groups can be seen from Fig. 38. He 

 derives the Metazoa from the Ciliophora. In the ciliates there is 



