102 THE INVERTEBRATE PHYLA 



becomes the anus; the mouth develops in another position. 

 Grobben then divided the phyla in the following fashion. 



Protostomia Deuterostomia 



Scolecida Chaetognatha 



Molluscoidea Echinodermata 



Mollusca Enteropneusta 



Annelida Tunicata 



Arthropoda Acrania 



Vertebrata 



By Scolecida, Grobben meant the Platyhelminthia, Entoprocta, 

 Aschelminthia and Nemertini. In the term Molluscoidea he 

 included the Phoronidea, Ectoprocta and Brachiopoda. Sometimes 

 the group Molluscoidea was referred to as the Tentaculata. 



Let us first of all consider the validity of these two major groups, 

 the Protostomia and the Deuterostomia. In the Protostomia the 

 situation is not as clear cut as the classification might suggest. 

 Thus in the platyhelminthes, the blastopore closes in Convoluta 

 and Planocera and the mouth is a new formation (there is of 

 course no anus in the platyhelminthes). In the polyclads the 

 original blastopore closes and disappears but the pharynx develops 

 near the site of the erstwhile blastopore. In the Tardigrada the 

 blastopore does not develop. In the Entoprocta the blastopore 

 closes and a new mouth and anus develop. In the Annelida the 

 situation varies according to the animal studied. In Nereis and 

 Podarke the blastopore forms the mouth and anus in the required 

 manner. In Pomatoceros the blastopore forms the mouth but the 

 anus is a new formation. In Capitella, Ctenodrilus and Saccocirrus 

 the blastopore closes and the mouth and anus are new formations. 

 In the oligochaete Dendrobaena the anus is a new formation and it 

 is not derived from the blastopore. 



In the Arthropoda the situation is much the same. In Peripatus 

 capensis the blastopore after a brief closure opens again to form 

 the mouth and anus. In some crustaceans such as Caridina the 

 anus develops some distance away from the blastopore region 

 whilst the mouth develops as a new formation unrelated to the 

 blastopore. In Astacus the proctodeum arises from the region near 

 the site of the blastopore. 



