8 C. 0. Whitman 



lateral cells , called para-polarcells by Van Beneden. The cau- 

 dal extremity is also formed of two cells , which , in young individuals 

 at least , generally have a dorso- ventral position — a position at rig-ht 

 angles to tbat of the parapolar cells. 



The calotte, and the parapolar cells together with all the other 

 external cells of the body, form the ectoderm. which is everywhere in 

 immediate contact with an internal axial cell , as Van Beneden has 

 already clearly shown. This axial cell may be said to represent the 

 endoderm , in view of its position and embryonic history. In origin, 

 structure , and general appearance , it is the same in each species , and 

 need not therefore occupy our attention in the foUowiug descriptions. 

 The ectoderm furnishes all the diagnostic characters: the number of 

 cells composing the calotte determines the genus ; while the size of 

 these cells, both relative and absolute , their shape , relative position, 

 axial relations, the whole nnmber of ectodermal cells, the leugth of adult 

 individuals, the form of the parapolar and caudal cells, etc., will supply 

 the means of distinguishing species. 



The genus Dict/ema liöW. ^ as already remarked, includes those 

 species in which the calotte consists of eight cells, in two sets of four 

 each. The octamerous calotte shows in most species a more or less 

 decided tendency to obliquity, which as a rule increases with age, and 

 often becomes so pronounced in the adults of some species, that all the 

 eight cells take part in forming the ventral face. This is evidently an 

 acquired character , since up to the time of birth , and often mnch later, 

 it is entirely absent. Van Beneden goes altogether too far when he 

 States (p. 17) that this obliquity is entirely due to a difiference in size 

 betweeu the dorsal and ventral cells. The explanation of the origin of 

 this peculiarity appears to me to lie in quite another direction. In the 

 youngest individuals , the calotte exhibits a symmetry so complete that 

 it is difficult to distiuguish between the dorsal and the ventral side. At 

 this stage there are obviously two planes coincidi ng with the axis of the 

 body — a vertical and a horizontal oue — , which divide the head into 

 two like parts. The junction of the calotte with the body forms a ver- 

 tical plane cutting the main axis at right angles. This form of the polar 

 calotte, in which the line formed by the intersection of the two dividing 

 planes is a direct Prolongation of the axis of the body , may be called 

 orthotropal. The orthotropal calotte passes into a plagiotropal form, 

 by an unlike güowth, not between the dorsal and ventral polar cells, but 

 between the dorsal and ventral sides of the body. The dorsal side elon- 

 gates more rapidly than the ventral, and the dorsal polar cells are thus 



