THE MOST PRIMITIVE METAZOA 59 



structure of a choanoflagellate cell. This protozoan had in many 

 ways the structure that one might expect to find in an ancestral 

 sponge and it has usually been figured as such in textbooks of 

 zoology. 



Proterospongia is not a common protozoan. Recently Tuzet 

 (1945) has investigated the antecedents and morphology of this 

 protozoan and she decided that in fact Saville Kent was the only 

 person ever to have definitely seen Proterospongia and that what he 

 saw was not a protozoan but a small fragment of an actual sponge. 

 Tuzet concludes, " Pour nous. ... La Proterospongia de Saville 

 Kent n'est pas autre qu'un corps de restitution d'Eponge d'eau 

 douce." Proterospongia is nothing more than a restitution body 

 of a fresh- water sponge. If this is true it is not surprising that 

 Proterospongia has many sponge qualities. On the other hand 

 Grondtved (1956) has described a new species of Proterospongia, 

 P. dybsoensis, in which there are three to ten cells arranged in a 

 linear row per colony. These colonies were often found in very 

 large numbers, up to 2,300 colonies per litre of water, and 

 Grondtved thought that they might be fragments from a larger 

 colony except for the fact that they were all so much alike. There 

 does seem to be quite a considerable difference between the row of 

 three to ten choanoflagellate cells embedded in a gelatinous 

 common envelope and Proterospongia as described by Saville Kent, 

 where the choanoflagellate cells migrated into the interior of the 

 colony and took up amoeboid structure. For this reason it is not 

 clear whether the new species rightly can be placed in the genus 

 Proterospongia . 



(2) From the Volvocinae 



There are certain resemblances between the embryonic develop- 

 ment of Volvox and that of certain sponges. Duboscq and Tuzet 

 (1937) showed that in Grantia the embryo developed inside a 

 membrane. The blastula is made up of two types of cells, 

 flagellate ones and non-flagellate ones. In the blastula all the 

 flagellate cells point inwards at first but during the course of 

 development the blastula turns inside out so that the flagella now 

 point outwards. This phenomenon is called inversion and is 

 shown diagrammatically in Fig. 19. The larva is then liberated 

 as an amphiblastula larva with the flagella at one end. The other 



