520 LECTURE XX. 



(excepting those of the Florideae and of certain Fungi, which 

 are better termed spermatia), and in some cases by plants, 

 such as Volvox, Pandorina, etc., and some Bacteria during a 

 longer or shorter period of their life. As a rule the ciliated 

 zoospores possess no cell-wall, but in some cases it is present 

 (p. i) : permanently motile plants, such as Volvox, etc., possess 

 cell-walls. 



Locomotion is also exhibited by Diatoms, Desmids and 

 Oscillatorias, but the mode in which it is effected is not fully 

 understood. According to some observers (M. Schultze, 

 Engelmann) the movement of Diatoms and Oscillatorias is 

 effected by means of the layer of mucilaginous substance 

 which invests the organism and which they believe to be of a 

 protoplasmic nature; Schultze even went so far as to suggest 

 that filaments of protoplasm are protruded through the 

 Diatom-frustule, along the median line (raphe) which act as 

 pseudopodia or temporary cilia. According to others (Nageli, 

 Dippel, Borscow, Mereschkowsky, Hansgirg) the movement is 

 the result of osmotic processes taking place between the cells 

 and the surrounding water. The nature of the movement 

 throws no light upon the subject ; the Oscillatorias revolve on 

 their own axes whilst moving forward, and this has been found 

 to be true of Diatoms in certain cases at least. It is admitted 

 on all hands that these organisms can creep over solid bodies, 

 but it has been denied by Cohn and others, that they can 

 swim: Nageli, Pfitzer, and others have, however, observed it. 

 The power of creeping rather suggests the existence of some- 

 thing like pseudopodia, whereas the swimming seems to 

 support the osmotic theory. It must be stated, however, that 

 nothing like pseudopodia has ever been detected, and that the 

 protoplasmic nature of the mucilaginous investing layer has 

 not been established. 



With regard to Desmids, a remarkable form of locomotion 

 has been described by Stahl in Closterium moniliforme : the 

 elongated cell attaches itself at one end, then swings itself 

 over and attaches itself by the other: each time this is repeated 

 the organism moves the length of its body. It is probable, 

 from Stahl's observations, that Desmids can swim. 



