Unicellular and Multicellular Animals 



a type it is a comparatively easy step to the Magosphaera described 

 by Haeckel and illustrated in Fig. 27. This consists of a simple 

 ball of ciliated cells, which reproduces by the occasional breaking 

 away of an individual member, which divides and redivides until 

 a new sphere is produced. Unfortunately this animal has only 

 once been discovered, and many hold that it has not been suffi- 

 ciently investigated. No other of the same type is known. 



If we turn to the plant kingdom, however, we find a com- 

 paratively common organism of somewhat similar form. This is 

 the Volvox, a plant which consists of some thousands of cells, and 

 reaches a size of about a pin-head. It has the form of a hollow 

 sphere, the wall of which is one cell thick, and the cavity of which 

 contains only water. The cells bear whip-like cilia on their outer 

 surface, by whose means the organism is able to move, swimming 

 by a rotary motion round a definite axis. The individual cells 

 are separated by layers of a gelatinous substance, through which, 

 however, pass connecting strands of protoplasm. The cells, of 

 course, contain the green colouring matter common to plants in 

 general. Distributed among the ordinary cells occur a few that 

 are distinguished by their larger size, and by the fact that they 

 lack cilia. These are the special reproductive members of the 

 colony. When the Volvox reaches maturity, these cells begin to 

 divide, and form new growths which take the form of hollow sacs, 

 which project into the cavity of the parent sphere. Later they 

 separate from the wall of the parent, and begin to move about, in 

 the internal cavity, by means of the cilia which they have developed . 

 Finally the parent breaks up and dies, and the progeny are set free 

 to commence life for themselves. 



The fundamental importance of this type is that we have 

 already a division of the life activities. The majority of the 

 cells are concerned in the nutrition of the individual as a whole. 

 These ultimately perish. A minority, however, are fed and pro- 

 tected by them, and these in return secure the perpetuation of 

 the race. This division into a mortal ' body ' portion and an 

 immortal reproductive portion is the first and most important 

 division of the life activities, whether in the animal or in the 



37 



