ORIGIN OF THE METAZOA 41 



Another interesting colonial dinoflagellate is the parasitic form 

 Haplozoon. This is found in the gut of polychaetes. It forms 

 first of all a small cell which attaches to the gut of the host by 

 means of a spike and some filamentous pseudopodia. This cell 

 absorbs food from the polychaete and at a later stage divides. 

 The results of division do not detach but instead remain in contact 

 so that a colonial form of up to several hundred cells is soon 

 formed, the number of cells differing from species to species. 

 These cells can form a three-dimensional mass with small spaces 

 between the cells through which food particles can be transferred 

 (Fig. 12). 



Fig. 12. Colonial dinoflagellates. Haplozoon was originally placed 

 in the Mesozoa but its spores have typical dinoflagellate structure 

 and it is now considered to be a colonial dinoflagellate. (From 



Grasse after Dogiel.) 



The relationship of Haplozoon to the Dinoflagellata is not 

 clear at first sight. In fact Haplozoon is so much like a metazoan 

 that when it was discovered by Dogiel in 1906 he placed it in the 

 Catenata, a new group of the Metazoa. It was not until the work of 

 Chatton (1920) that it was shown that the cells at the posterior 

 end of Haplozoon detached and developed into four small spores, 

 each of which had characteristic dinoflagellate structure. It is for 

 this reason that Haplozoon is placed in the Dinoflagellata. This 

 reasoning can, if carried to its illogical conclusion, lead one into 

 difficulties. Thus Duboscq and Grasse (1933) have shown that 

 the mammalian spermatozoan is very much like a protozoan of the 

 group Bodoines, yet I doubt if anyone would like to place Man in 

 the Protozoa on account of his male gamete ! 



Colonial Protozoa are also found in the other classes. In the 

 Ciliphoroa, Anoplophrya forms chains of cells, whilst Carchesium 

 and Zoothamnion form branching colonies (Figs. 13 and 14). 

 In Zoothamnion the myonemes run throughout the length of the 

 colony so that if one part contracts then all the rest contracts. In 

 Carchesium the myonemes are restricted to each unit so that they 



