8 



STUDIES IN THE MORPHOLOGY, TAXONOMY, AND ECOLOGY OF THE PERIDINIALES 



tral part, a wide right limb lying posterior to the right 

 sulcal plate, and a narrow or wide limb on the left side 

 running along the left side of the left sulcal plate. In 

 some species (P. truncatum) it assumes a size equal to 

 that of some of the major body plates. It bears some of 

 the elements of the more complicated list systems of 

 the Peridiniales. 



The left sulcal plate (Is, fig. 1) is present in all ge- 

 nera. It has undergone little evolution of shape, but 

 varies in position in the sulcal tabulation. It is always 

 anterior to the posterior plate, however, and on the left 

 side of the area, and it always borders the flagellar 

 pore. It may form the entire left edge of the pore, as 

 in Peridinlum and Goniodoma ; it may form the posteri- 

 or edge and part of the left edge of the pore, as in Gon - 

 vaulax ; or it may form only the posterior edge of the 

 pore as in Spiraulax and Ceratocorys ; or only part of 

 the posterior edge, as in Acanthogonyaulax . 



In only one genus, Acanthogonyaulax . is there a left 

 accessor y sulcal plate (la, fig. 1). This lies not contig- 

 uous ta the left sulcal plate, but anterior to the flagel- 

 lar pore on the left side of the sulcus. 



On the right side of the ventral area there may be 

 from one to three plates between the anterior and the 

 posterior; viz., one in most species of Peridinium . two 

 in Goniodoma . Ceratocorys . and Peridinium pallidum , 

 and three in Gonyaxilax . Acanthogonyaulax , and Spirau- 

 lax. When only one plate is present, it is the right sul - 

 cal plate (rs, fig. 1). Anterior to this plate there may 

 be a right accessor y sulcal p late (ra, fig. 1); posterior 

 to it, an intercalar y sulcal plate (i, fig. 1). Either the 

 right or the right accessory, or both, borders the pore, 

 but the intercalary never does. Sometimes the right 

 is not in contact with the pore, as in Goniodoma and 

 some species of Ceratocorys . In such cases the right 

 accessory and the anterior sulcal plates form the right 

 edge of the pore. In addition to these seven plates there 

 is frequently an eighth, the p osterior accessor y sulcal 

 plate (pa, fig. 1). This lies between the posterior edge 

 of the pore and the posterior plate as in Peridinium . or 

 it may be squeezed out of contact with this plate as in 

 Gonyaulax pacifica . It occurs only in these two genera. 



Discussion . It is obvious that there is a great 

 diversity in the tabulations of the ventral areas of the 

 various genera in the Peridiniales. In spite of this 

 diversity, however, a common basic plan is evident. 



After a careful comparison of the ventral areas of 

 the various genera, a type of tabulation was devised 

 which may represent an approximation of the ancestral 

 condition (fig. lA). This reconstructed ventral area is 

 quite similar to that of Goniodoma with the major ex- 

 ception that its posterior sulcal plate does not touch the 

 flagellar pore. Since all genera except Goniodoma have 

 this plate removed from the pore, Goniodoma must be 

 considered aberrant in this respect. This primitive 

 ventral area is thus composed of five plates: an anteri- 

 or sulcal, a posterior sulcal, a right and a left sulcal, 

 and a right accessory sulcal. These plates are approx- 

 imately equal in size and all border the pore except the 

 posterior sulcal. The sulcal ring is complete. 



If this reconstruction of the ancestral ventral area 

 is used as a key, a comparison of this area in the vari- 

 ous genera yields the following results: 



Goniodoma : This is, without doubt, the most prim- 

 itive of the genera investigated, since its ventral area 



is the simplest of those studied, being different from the 

 ancestral condition only in having the posterior plate 

 touching the flagellar pore. Its ventral area is very 

 simple in outline and contour; the flagellar pore is ful- 

 ly exposed; there are no internal skeletal structures; 

 and the sulcal lists are poorly developed. There are 

 only five plates. These are subequal in size and show 

 no complex contours. The sulcal ring is complete. 



Ceratocorys : The genus Ceratocorys is evidently 

 closely related to Goniodoma. The sulcal plates, also 

 five, are simple in structure, and apparently are homol- 

 ogous with the five present in Goniodoma . There has 

 been a narrowing of the sulcus, however, and a slight 

 enlargement of the posterior sulcal plate. The sulcal 

 ring has broken on the left side and the posterior sulcal 

 plate has taken up a position posterior to the right sul- 

 cal plate so that it alone forms the posterior end of the 

 area. The last two characters are carried through the 

 other genera except that the ring is not broken in Peri - 

 dinium . Thus although Ceratocorys is closely related 

 to Goniodoma . it represents a definite advance over that 

 genus. 



Gonyaulax . Acanthogonyaulax . Spiraulax: These ge- 

 nera show striking similarities. Their ventral areas 

 are greatly elongated and sigmoid in outline, with great- 

 ly enlarged posterior sulcal plate. The number of plates 

 has increased to six or seven. The sulcal ring is bro- 

 ken. The sulcus proper is a narrow groove occupying 

 only part of the ventral area. All the plates of the 

 primitive area are present, and in addition, there is an 

 intercalary lying on the right side between the right and 

 the posterior sulcal plates. In addition to these, Gon - 

 yaulax pacifica has a posterior accessory plate just 

 posterior to the pore. In Acanthogonyaulax there is a 

 left accessory sulcal plate just anterior to the pore, and 

 the anterior sulcal plate is unique in being removed 

 from the pore. 



Peridinium. Ceratium : These two remaining gene- 

 ra are widely divergent from the previous genera and 

 each of them is unique in regard to its sulcal structures 

 In Peridinium the primitive number of external plates, 

 i.e., five, has been retained, except in P. pallidum ; and 

 the sulcal ring is unbroken. Each sulcal plate, however, 

 is singularly modified, particularly the posterior, and 

 internal sulcal structures have developed. Sulcal lists 

 are prominent and the sulcus proper is deeply embed- 

 ded in the left side of the area in such a way that it is 

 not exposed to view. Ceratium is characterized by the 

 development of a wide and large flat ventral area ex- 

 tending over the ventral side of the body, and covered 

 by hyaline plates. This type is to be found nowhere else 

 in the Peridiniales. 



It can be concluded from the above studies that 

 Goniodoma represents the most primitive genus inves- 

 tigated; that Ceratocorys is closely related to it but is 

 somewhat more advanced; that Gonyaulax , Acanthogon - 

 yaulax . and Spiraulax are decidedly more advanced and 

 in addition are closely related to each other; and that 

 the genera Peridinium and Ceratium are highly special- 

 ized and developed along divergent lines. 



Thus, the genera cannot be arranged in a single 

 evolutionary series. Evolution has apparently prog- 

 ressed in divergent directions within the order and these 

 divergent directions are expressed by an apparently 

 natural grouping of the genera. 



