OF THE PHYTOZOA. 139 



the other, the quadrisection takes place as in the former case ; but when, on the 

 other hand, one dimension has become predominant, the division into four takes 

 place in accordance with the same law as the original di\ision into two. 



'^ The method of division fii^st described is always met with in the early 

 periods of an infusion, Avhich are most favourable to the development of the 

 creatures. Towards the end the latter mode alone occurs. This phenomenon 

 was so remarkable that, on the first occasion of my examining an infusion 

 towards the close of its action, I imagiaed that I had at first misunderstood 

 the mode of division. 



" Under certain circumstances the individuals pass to a state of rest. They 

 are then completely filled with the starch-like granules, so that the nucleus 

 only appears as a reddish spot. The substance of the body becomes spherical, 

 and invests itself with a membrane which is frequently of considerable thick- 

 ness (fig. 7). In this state I have never observed them to undergo any divi- 

 sion or any other change ; and when diied the cysts still retain their contents. 

 When clear water is poui'ed over them they do not return to hfe, but would 

 probably do so in a fermenting infusion. 



'' The mode in which the swarming individuals arrive at this state of repose 

 appears to be as follows : — The filaments are gradually shortened, their sub- 

 stance collecting at the free extremity in the form of a small knob, until at 

 last the filiform portion entirely disappears, and, in place of the filaments, two 

 vesicles are seen at the anterior extremity of the investing membrane. I 

 have obsei-ved a similar contractibility of the substance of the filaments in a 

 Bodo which is most nearly allied to Bodo grandis, Ehrbg. As this possesses not 

 three filaments only, as seen by Focke {Elir. p. 34), but often as many as five, 

 the vesicles produced in this manner cannot easily be overlooked. I cannot, 

 however, state with certainty whether all the individuals which undergo this 

 change invest themselves Tvith cysts. "WTien infusions containing Pdytoma 

 are diied slowly, individuals with the vesicles just described are found in the 

 deposit, but no cysts ; and it is not impossible that such indi\-iduals may assist 

 in the continuation of the species in some other way." 



After some valuable notes on other Infusoria, Schneider' concludes his 

 history of Polijtoma by the following arguments for its animal nature : — 



*' That Polytoma is an animal may be maintained upon two grounds. 



" 1. The constitution of the investing membrane. — As soon as the starch-like 

 granules have been destroyed by the long action of concentrated sulphuric 

 acid, no part of the creatm^e is coloured blue by iodine. Now we have no 

 more reason for believing that the vegetable cell-membrane must necessarily 

 consist of cellulose, than that the animal cell-membrane should not consist 

 of that substance, so that we are still compelled to seek for other characters 

 for their distinction. These would be — 



" 2. The contractile spaces. — A statement of Cohn has ceriainly rendered 

 it doubtful whether the occurrence of these is henceforward to be regarded as 

 an essential indication of an animal nature. He says, ' On the other hand, 

 certain genera of Algae exhibit a stage of development in which, in external 

 form, in the absence of a cellulose membrane, in the distinct existence of 

 ciliaiy organs of motion, red eye-like spots, vacuoles, and, according to a very 

 recent discovery, of interned pulsating spaces, they undoubtedly appear very 

 similar to the Astomatous Infusoria.' If these pulsating spaces occur only 

 in unicellular Algae provided with cilia, these perhaps should properly be re- 

 stored to their place amongst animals, notwithstanding the subsequent ap- 

 pearance of cellulose-membrane upon them. But if they occur in the swarm- 

 cells of the Confervae, they certainly cease to be a characteristic of animal 

 nature. Thus, if we are not yet in a position to refer Polytoma with perfect 



