64 



Peridiniede 



is of a denser character than the more central part, and as a rule contains 



many large granules. The central region, 

 on the other hand is much more finely 

 granulate, and contains the nucleus, the 

 cell-sap vacuoles, and a contractile vacuole. 

 Sometimes the distinction between these 

 zones is not very distinct, but in other 

 species the layers are sharply separated. 

 There is also a thin layer of cytoplasm on 

 the exterior of the cell-wall which is directly 

 connected with the inner cytoplasm by the 

 fine strands passing through the pores of 

 the wall. The nucleus is spherical, oval, 

 or reniform in shape, and is located in 

 most forms in a fairly central position, 



Fig. 46. A, ventral view of Amphidoma . . . . 



WcomcaKofoid .to show the protoplast although it may be sometimes anterior or 



and skeletal inclusions; ch, chro- poster i or { n position. On the whole, it is 

 matophore; n, nucleus; k, mtra- 



cellular skeleton ; v, vacuole. x 750. relatively large and undoubtedly primitive. 



'~ d ' Jt exhibits a striation due to the presence 

 of rather numerous parallel threads. The 

 latter are sometimes rather stout and they consist of nuclear rods enclosed in 

 nuclear tubes. There may be one or more nucleoli present. 



In some of the Peridiniaceae there is a red pigment-spot (the so-called 

 ' eye-spot '). It is disc-shaped in Olenodinium neglectum Schtitt and horse- 

 shoe-shaped in GL cinctum Ehrenb. It agrees in all essential details with 

 the pigment-spot of the Flagellata, consisting of a protoplasmic ground- 

 substance in which are embedded small rod-shaped masses of hsematochrome. 

 In the genus Peridinium the pigment-spot has only been observed in the 

 two species, P. quadridens Stein and P. balticum (Lev.) Lemm. Zacharias 

 has stated that P. quadridens possesses two pigment-spots, one in each half 

 of the cell. 



The longitudinal flagellum is generally longer than the cell, and is 

 directed backwards, corresponding to the trailing flagellum of many of the 

 Flagellata. Sometimes it lies in the longitudinal furrow, but more often it 

 stands out obliquely from the cell. Its movements are usually whip-like, 

 but it may spirally contract and then suddenly unroll itself. In certain 

 species of Ceratium and Podolampas it has also been observed to withdraw 

 itself entirely within the flagellar pore, and occasionally in Ceratium tripos 

 and C. cornutum two longitudinal flagella have been seen. 



The transverse flagellum, which is at all times difficult to demonstrate, 

 lies concealed in the transverse furrow, and in some forms the concealment is 

 all the more complete by the development of ridge-like expansions at the 





