THE MOST PRIMITIVE METAZOA 



71 



therefore more primitive than the coelenterates. Our conclusion, 

 therefore, is that the situation is not at all clear. 



(2) The Mesozoa 



The Mesozoa are a group of parasitic animals of very simple 

 structure. They are multicellular and usually take the form of a 

 solid mass of cells with one or more internal cells. These internal 

 cells are not digestive in function but instead play a part in the 

 reproduction of the animal. 



The mesozoans in some ways correspond in structure to a solid 

 blastula and it has been suggested by some writers such as van 

 Beneden (1876) and Hyman (1940) that the Mesozoa are a 

 primitive group, or even the most primitive group, of the Metazoa. 

 On the other hand the Mesozoa are all internal parasites. Thus 



(a) Ciliated larva 



(b) Female 



(c) Male 



Fig. 25. Mesozoan structure. Rhopalura. 



(a) Ciliated larva. (From Hyman after Atkins.) 



(b) Adult female. (From Hyman after Caullery.) 



(c) Adult male. (From Hyman after Caullery.) 



