iv] THE SECOND GRADE 109 



be no doubt that their function is reproductive, 

 designed to spread the race to other hosts. 



That is the simplest form: thence to the most 

 complex an interesting series may be traced, through 

 species where a few of the hinder cells divide in such 

 a way that the animal's posterior end is a plate, not a 

 mere row, of cells, then up to others where this state 

 of things begins much earlier, so that the plate is 

 broadly wedge-shaped, and finally to forms where the 

 hinder cells divide in all three directions of space, and 

 the posterior end is large and club-shaped, several 

 layers in thickness (Fig. 8). In the front half of the 

 body, little openings exist between cell and cell, which 

 serve to pass food-substances down from the " head " 

 into the other cells. When these are full-fed, they 

 close themselves off from their neighbours and pre- 

 pare themselves for their reproductive destiny. 



The ancestry of these curious creatures is almost 

 certainly to be sought in another group of plant-like 

 unicellular flagellates, the Peridineae. These are two 

 forms which serve to bridge the gap a large one 

 between the active free Peridineae and the parasitic 

 multicellular Haplozoon. 



The first, Gymnodinium pulvisculus (Fig. 9, a), is 

 also a parasite but an external one: it is found at- 

 tached to the skin of various pelagic creatures by a 

 stalk or bundle of sucking pseudopodia like those of 

 Haplozoon. So it thrives till it is full grown : then, 



