39 



give rise to aggregates of small elongated flagellate male 

 cells or microgametes, which are set free and pass into 

 the cavity. The microgamete swims towards a macro- 

 gamete, enters it and fuses with it (syngamy), and the 

 result is a fertilised egg or zygote. The zygotes have thick 

 envelopes with spines. They remain unchanged through 

 the winter, and in spring develop new colonies (asexually) 

 by repeated fission. 



On what Grounds may Volvo x be regarded as intermediate be- 

 tween (a) Animals and Plants, (b) Protozoa and Metazoa ? 



VOLVOX has chlorophyll (chromatophores). By means of 

 these it decomposes CO. 2 and forms a starch ; therefore it 

 is holophytic like a plant. It is also plant-like in having 

 pyrenoids with paramylum granules. But it moves actively 

 in search of food, and it has flagella and contractile vacuoles, 

 and these are animal features. It is therefore intermediate 

 between Plants and Animals. 



The Volvox individuals are unicellular protozoans ; 

 but their differentiation (somatic cells and reproductive 

 cells of two distinct kinds) and the grouping together as 

 a colony appear to indicate transition and approach to 

 many-celled Metazoa. Volvox may therefore be regarded 

 as intermediate between Protozoa and Metazoa. 



What is Proterospongia and what is its Zoological Interest ? 



One of the Choanoflagellata. It forms colonies com- 

 posed of two structurally distinct kinds of cells, amoebo- 

 cytes and choanocytes. The choanocyte has a flagellum 

 surrounded by a collar-like extension of the cytoplasm. 

 The collar is retractile, and to it adhere the food-particles 

 brought by the water currents caused by the activity of 

 the flagellum. Similar choanocytes are characteristic 

 of Sponges. (See page 67.) 



