Peridiniaceas 



69 



across the girdle into the antapical half between 4"' and 3'", 3'" and 2"" 



2'" and 2"", 2"' and I"", V" and I"" ', and so back again to the ventral area 



(see fig. 49). The so-called ventral plate, which forms the ventral area, is 



thus obliquely divided by the fission-plane into 



an anterior and a posterior portion. The position 



of the fission-plane with reference to the skeletal 



plates was correctly given by both Biitschli ('85) 



and Bergh ('86), but only Kofoid ('07 c and D) 



has given a complete and correct account of the 



number and disposition of the plates. 



As the daughter-cells separate after the 

 division is completed, their exposed proto- 

 plasmic surfaces become so moulded as to 

 complete the normal outward form of the organism. 

 They are at the same time clothed with a thin 

 wall which soon shows the plates and sutures 

 characteristic of the species. 



In Peridinium, and other allied genera, the 

 old thick wall is frequently thrown off, and the 

 protoplast escapes in a new thin wall. In this 

 condition cell-division may occur (Gonyaulax), 

 often in a transverse plane. Sometimes the 

 protoplast on its escape becomes enveloped in 

 a wide gelatinous envelope, under cover of which 

 the division takes place (fig. 50 0). The ecdysis 

 of the old cell-wall (figs. 50 D and 517) often results from the bursting of the 

 girdle, the two valves falling apart (Glenodinium spp., Peridinium aciculiferum); 

 by the loosening of certain plates only, as in Peridinium Willei, where the 

 apical and some of the precingular plates are set free ; or quite irregularly, as 

 in several species of Peridinium. 



A method of multiplication has been observed in Peridinium anglicum 

 by the production of thin- walled, non-resting ' cysts ' (G. S. W., '09). The 

 protoplast becomes rounded, develops a new cell-wall, and then throws off 

 the old articulated wall. The development now proceeds in one of two 

 ways : (1) The rounded ' cyst ' may divide at once into two cells each of 

 which rapidly becomes an adult individual with a wall of typical plates. 

 Sometimes each cell divides again before the formation of the tabulated wall, 

 so that four daughter-cysts are formed from the original ' cyst.' (2) The 

 ' cyst ' may give rise to a motile cell, with a thin plasma-membrane, which at 

 once escapes from the cyst- wall and begins to divide while slowly swimming 

 about. Such cells generally divide transversely several times, and the final 

 generation of daughter-cells become typical adult individuals. During these 



Fig. 49. Ceratiumfurca (Ehrenb.) 

 Duj. x 350. A, ventral view ; 

 B, dorsal view. 1 4, girdle 

 plates; 1' 4', apical plates; 

 I" 4", precingular plates ; 

 V" 5'", postcingular plates ; 

 V" and 2"", antapical plates ; 

 v.p., ventral plate. The sutures 

 shown by the double line indi- 

 cate the line of fission. (After 

 Kofoid. ) 



