1918] Barroivs: Skeletal Variations in the Genus Peridinium 421 



to an attempt at adustment of buoyancy to density in flotation or to 

 the imparting of a spiral method of locomotion, making rectilinear 

 progression possible, or to such an upsetting of the body of the dino- 

 flagellate as it falls through the water as to bring it to present as 

 great an area as possible to the direction of falling and thus to reduce 

 the rate of falling by increasing the amount of resistance of the water 

 to the falling body. Solid antapical horns may also serve some of these 

 same purposes or be coupled with the function of locomotion in some- 

 what the same way as the lists on the margins of transverse and longi- 

 tudinal grooves are related to the activity of the flagella by confining 

 the currents of water set up by the flagella. 



Thus it seems to be the epitheca, or the anterior end of the organ- 

 ism, which reflects the active internal forces of the organism and the 

 hypotheca, which possibly reflects certain superficial modifications 

 stimulated more directly by the environment. This conclusion is 

 somewhat in accord with the theory of gradients recently put forward 

 by C. M. Child, according to which we should expect the rate of meta- 

 bolism to be higher in the anterior than in the posterior end of organ- 

 isms, and hence the initiative for profound morphologic modification 

 more pronounced here than in the posterior region. 



The number of sides of each polygonal plate of the skeleton varies 

 in this genus from three to six, the more common number being five. 

 Only the plates 1" and 1" in species of Paraperidiniuni and plate 1" 

 of Metapcridinmm (dextrad) are triangular. These are presumably 

 plates of rather recent phylogenetic origin. Hence a triangular shape 

 i.s believed to be an undeveloped or primitive condition in the history 



of a plate. 



The reticulated surface markings also enclose polygonal areas, 

 bounded by straight or nearly straight sides, among which there are 

 found comparatively few triangular areas, the areas being usually 

 five- or six-sided. However, none of these plates or reticulated areas 

 is equilateral or symmetrical. This condition is in contrast to the 

 frequent condition in other animals of repeated hexagonal elements, in 

 which the elements are nearly if not quite regular, such as the omma- 

 tidia of a compound arthropod eye, the comb cells of a bee hive, and 

 the framework of the skeleton of many Radiolaria. 



There is every indication that not only the formation of plates in 

 the shell but also the size of these plates is a matter of internal regula- 

 tion. This appears from the circumstance that after ecdysis the shell 

 is reformed with plates presumably in the original pattern. While 



