iro 



THE EVOLUTK^N TilEOHY 



aiv can-iod ovtT as a heritable possession of the polyps IVoin ()ii(> 

 generation to another, ainl in a very interestino- manner, namely, by 

 means ol' the ef'^s, ami I'V these alone. As Hamann has shown, the 

 zooehlorella> mi^n-ate at the time when an eui;- is formed in the onter 

 layer of the body of the polyp (Fig. 35) from the inner layer outwards, 

 piercing through the supporting layer between them {d) and pene- 

 trating into the egg (B, Eiz). They make their way only into the egg, 

 not into the sperm-cells, which in any case are too small to include 

 them. Thus they are absent from no young polyp of this species, 

 and it is easy to understand why earlier experimental attempts to 



rear colourless polyps from eggs could 

 never succeed even in the purest water. 

 Quite similar green alga? live in 

 symbiosis Avitli unicellular animals, as, 

 for instance, with an amoeba (Fig. 36) 

 and with an Infusorian of the genus 

 Burtaria. In the Zoological Institute 

 in Freibiirg there is a living colony 

 of a green amceba and a gi-een 

 BurtavM, both of which came from 

 America, sent to us some years ago by 

 Professor Wilder, of Chicago, inside 

 a letter with dried Spltagnum, or 

 bog-moss. The plants came from stag- 

 nant water in the Connecticut valle}' 

 in Massachusetts. That in this case 

 the zoochlorellse are of use to the 

 animals within which they live, not 

 only by giving off oxygen, but also by 

 yielding food-stuff, has been proved by A. Gruber, who bred the tAvo 

 green species for seven years in pure water which contained no trace 

 of an}" kind of organic food for them. Nevertheless, they multiplied 

 rapidly, and still form a green scum on the walls of the glass in which 

 they are kept. They only die away when they are kept in the dark, 

 where the algaj are unable to assimilate ; then one green cell after 

 another wanes and disappears, and, in consequence, their hosts also die 

 from the double cause of lack of oxj^gen and lack of food. 



Even in this case the symbiotically united oig^anisms have not 

 remained unaltered. The algse at least differ from others of their 

 kind in their power of resistance to living animal protoplasm. They 

 are not digested by it, and we may infer from this that they possess 

 some sort of protective adaptation against the dissolving power of 



Fig. 36. A, Aiituha riridis. k, the 

 nucleus, cc, contractile vacuole, scld, 

 the zooehlorelUe. B, a single zoo- 

 chloiella under high power. After 

 A. Gruber. 



