76 PeridiniedB 



One of the great difficulties in the biological investigation of the 

 organisms embraced in the Peridiniese is their extreme sensitiveness to even 

 trifling alterations of environment. The slight changes to which they are 

 necessarily subjected during collection and microscopical examination are 

 sufficient to cause almost immediate death and degeneration. 



It is not possible to make any definite statements on the geographical 

 distribution of the Peridiniaceae with our present imperfect knowledge of the 

 group, but there is much evidence to show that many forms are only found 

 in the warmer oceans and that others are similarly restricted to temperate 

 areas. Among the freshwater forms, Peridinium aciculiferum appears to be 

 a northern type, and Ceratium hirundinella is without doubt the most 

 ubiquitous. 



A few fossil representatives of the Peridiniacese are known. The species 

 found by Ehrenberg ('36) in a siliceous rock of Cretaceous age from 

 Delitzsch in Saxony, and described as Peridinium pyrophorum, bears a 

 striking resemblance to the recent Peridinium divergens Ehrenb. 



Family Prorocentracese. 



This small group of the Peridinieas is characterized by the entire absence 

 of the transverse and longitudinal furrows. According to the morphological 

 interpretation put forward by Btitschli, Schiitt, and others, the cell is com- 

 pressed from the poles so that the longitudinal axis is. the shortest one. 

 It is ellipsoid, egg-shaped, or from the dorsal view the outline may be 

 lanceolate (Prorocentrum micans Ehrenb.). In all except Haplodinium, 

 in which the outer cellulose covering is structureless, the cell- wall is composed 

 of two watch-glass-like plates which cover respectively the anterior and 

 posterior halves of the protoplast. These two plates are precisely similar 

 and, as the girdle is absent, their edges are directly joined along the median 

 equatorial line, which is also the greatest circumference of the cell. In most 

 of the forms the plates are furnished with distinct pores, which may be 

 distributed over the whole of the wall or restricted to definite regions. In 

 the middle of the ventral surface is the flagellar pore, which in Cenchridium 

 projects inwardly into the cell-cavity as a tube. In the disposition of the 

 longitudinal flagellum the Prorocentraceaa differ from all the other Peri- 

 diniese. This flagellum is carried in front of the cell so that during the 

 progression of the organism through the water the body of the cell is dragged 

 after the longitudinal flagellum. The transverse flagellum is sometimes out- 

 standing, but more often vibrates close to the cell-wall along the equatorial 

 line. It is of about the same length as the longitudinal flagellum and 

 usually swings round its base, but does not extend for more than about 



