244 LECTUBES ON BIOLOGY 



one uniform mass : and in the midst of this common protoplasm 

 the prey lies defenceless. When the meal is finished the animals 

 separate once more, and each goes its own way until want of 

 food may chance to bring them together in another spot. 

 Thanks to this union, the Heliozoa are able to overpower even 

 animalcules which are far superior to them in strength and 

 ability. 



A still higher stage of development than the Vorticella 

 colonies has been reached by Pandorina morum, the small 

 flagellate belonging to the family of the Volvocidae, an organism 

 which stands on the borderland between the plant and the animal 

 world. 



In many parts of Europe the Pandorina may be observed in 

 any little pond. Sixteen cells have formed themselves into a ball 

 and produced by common labour a gelatinous cover which pro- 

 tects their bodies. In Pandorina all parts of the body are as yet 

 equivalent ; each cell is furnished with nucleus, protoplasm, con- 

 tractile vacuoles, light-sensitive pigment, chlorophyll, and two 

 flagellae which protrude from the gelatine-cover and serve as 

 organs of locomotion. Equal is also the work which each cell 

 performs for itself and the community, but in the fact that 

 the members of the colony already do common labour, undertake 

 the nutrition in common, and build the gelatine-cover in common, 

 lies the great progress as compared with the union of the 

 Vorticellse. What binds the Pandorina-cells so firmly together 

 that they no longer live as individuals is difficult to see, for 

 each cell is capable of performing all essential life-functions, 

 and would therefore undoubtedly be able to look after itself, yet 

 they are always found united in this manner. Strictly speak- 

 ing, Pandorina can no longer be regarded as a protozoon or 

 protophyton, for it represents already a higher multicellular 

 organism. 



As in Pandorina, in contrast to the multicellular organisms, 

 the single body-cells are as yet entirely equivalent, each indi- 

 vidual cell possesses therefore the faculty of reproduction and 

 of forming a new colony of sixteen cells. Moreover, repro- 

 duction proceeds exactly as in a typical protozoon. When a 

 Pandorina colony has been floating about for a certain time in 

 the water and the cells have become fully developed as the result 



