Pt 'oi 'ocentracese 



77 



one fourth of the circumference of the cell. Close to the flagellar pore there 

 are in some forms minute ridge-like projections of the cell, or in others more 

 or less prominent tooth-like structures, which may possibly be the rudiments 

 of the expanded borders of the transverse furrow. 



The protoplast is similar to that in the Peridiniacece, consisting of an 

 outer granular zone containing the chromatophores, and an inner more 

 hyaline mass with cell-sap vacuoles and a nucleus. In the inner part of the 

 protoplast there are also pusules (consult p. 66). The nucleus lies as a rule 

 in that part of the cell towards the nagellar pore. The chromatophores are 

 two in number, of a golden-brown colour, disc-shaped or expanded and lobed, 

 sometimes reticulated, and they may overlap so as to give the appearance 

 of one large parietal plate. In the larger chromatophores a pyrenoid may be 

 present, as in Exuviaella Itevis. 



Multiplication takes place by division in the plane of the equatorial joint, 

 that is, at right angles to the longitudinal axis of the cell. Each daughter- 



Fig. 56. A and B, Exuviaella marina Cienk., x 400 ; A, apical view; B, side view. (7 and D, 

 Prorocentrum micans Ehrenb., x400; C, apical view; D, side view. E, Cenchridium 

 globosum (Williams) Stein, apical view, x 300. (After Schiitt.) 



cell retains one of the two old wall-plates and becomes provided with a new 

 half- wall which is soon firmly joined to the old one. Resting-spores have 

 only been observed by Cienkowski in Exuviaella. They are described as 

 pyriform and attached by the pointed end to various Algse. 



There are only four genera of the Prorocentraceee, Cenchridium, Exuviaella, 

 Prorocentrum, and the recently described Haplodinium, all of which occur in 

 the marine plankton, although a few forms have been observed in brackish 

 water. 



Klebs ('12) has suggested that the orientation of the cell is not as 

 described by Schiitt, but that the suture may be longitudinal, the flagella 

 being anterior in their attachment. This interpretation certainly emphasizes 

 the remarkable similarity between Prorocentrum and some of the Crypto- 

 monads, with Haplodinium as a form more or less intermediate in character. 



