58 



THE MOST PRIMITIVE METAZOA 



(A). Proterospongia 



(B). Sphaeroeca 



Fig. 18. Colonial protozoa that in some ways resemble sponges. 



(A) Proterospongia. (After Saville Kent.) This is now believed 



to be a fragment of a fresh- water sponge. 



(B) Sphaeroeca; single choanoflagellate protozoan and the colonial 



form. (From Grasse after Lauterborn.) 



