Peridiniacese 



73 



If unfavourable conditions arise during cell-division, the process may be 

 suddenly arrested, and undetached division-stages may be formed. These 

 are not unlike certain of the presumed conjugation-states, and have some- 

 times been mistaken for such. 



RESTING-SPORES. At the close of the active vegetative period many 

 of the freshwater Peridiniacese are known to enter into an encysted condition 

 by the production of thick-walled resting-spores (figs. 500 and 54 E). Only 

 one resting-spore is formed from a single individual. When the period of rest 

 is of some length, as in Peridinium aciculiferum, where it extends over nine 

 or ten months (G. S. W., '09), the wall of the resting-spore is very thick. 

 Similar resting-spores are formed at any time in some species when the 

 environmental conditions become unfavourable, such as by a sudden change 



Fig. 54. Ceratium hirundinella 0. F. Miill. A and B, three-horned forms; C and D, four- 

 horned forms; E, three-horned form with resting-cyst. Z> is a ventral view; A C, and E 

 are dorsal views. All x 200. 



of temperature or by alteration in the chemical constituents in solution 

 in the water. In most species the resting-spores are ellipsoid or ovoid, 

 but in Ceratium hirundinella they are usually three-angled or four-angled 

 and somewhat twisted, and each angle is furnished with a short spine 

 (fig. 54 E). It is most probable that these resting-spores remain in the mud 

 at the bottom of the water until the conditions are again favourable for the 

 resumption of the active vegetative phase, but their development has not yet 

 been observed. 



In some of the marine forms the protoplast becomes rounded off, secretes 

 a wide gelatinous coat, to which the remains of the tabulated wall often 

 stick ; and it either rests in this condition as a ' gelatinous spore ' or divides 

 into a number (up to 128) of smaller cells which remain within the same 

 envelope. The further fate of these spores has not been traced. 



The occurrence of ' swarm-spores ' of the Gymnodinium-tyipc is known 



