CHAPTER XI 

 POLYZOA 



Classification adopted — 



' Phylactolaemata 



fCyclostomata 



Polyzoa Ectoproctaj (jyj^nolaemata ] Ctenostomata 



Icheilostomata 



Polyzoa Entoprocta 



The group of the small colonial animals known as the Polyzoa 

 includes two divisions, known respectively as the Ectoprocta and 

 the Entoprocta, about whose affinity with one another there is very 

 considerable doubt. Both groups agree in being colonial, in possessing 

 a ring of ciliated tentacles surrounding the mouth, by the action of 

 which they obtain their food, and in having the principal nerve 

 ganglion situated between mouth and anus on the surface which is 

 normally turned upwards. 



In the Ectoprocta the coelom is spacious and well developed, 

 and from its walls the genital cells are developed ; whilst the body 

 is divided into a posterior part (the zooecium) and an anterior introvert 

 (the polypide). The ring of ciliated tentacles surrounds the mouth 

 alone, and is therefore morphologically a metatroch. In all these 

 features the Polyzoa Ectoprocta resemble the Podaxonia. 



In the Entoprocta, on the other hand, the coelom is entirely 

 suppressed, except in so far as it is represented by the minute 

 cavities of the genital organs. There are distinct nephridia, ending 

 internally in blind ciliated ends ; the body is divided into an upper 

 cup -like part called the calyx, and a lower solid stalk. The ring 

 of ciliated tentacles surrounds both mouth and anus, and is, morpho- 

 logically, a prae-oral band or prototroch. 



Prouho (1892), Harmer (1896), Seeliger (1906), and CzwikUtzer 

 (1909) regard the two groups as closely allied, but Korschelt and Heider 

 (1892) regard them as totally distinct phyla. We shall deal with them 

 separately in this chapter, and, after having studied both, indicate 

 our opinion as to which side in this controversy has the greater weight 

 of evidence in its favour. We begin with the Polyzoa Ectoprocta. 



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