522 H. S. Jennings and George T. Hargitt. 



2. Similarities and differences of the various races in form 



and structure 



We have seen that the various races show permanent differences 

 in size, and that certain races (aureha) differ in the number and 

 structure of the micro-nuclei from others (caudatum). Are the 

 races identical in other repects? Are the small races merely minia- 

 ture replicas of the larger ones, so far as form and structure are 

 concerned? 



As is well known, certain diversities have been set forth as dis- 

 tinguishing the species caudatum from the species aurelia. Ehren- 

 berg's original description of caudatum as a separate species 

 ('38) was based on a difference in form, the posterior part of cau- 

 datum was said to be longer and more slender than in aurelia, 

 tapering (as shown by his figures) regularly backward from about 

 the middle to the posterior point. In aurelia on the other hand the 

 posterior half of the body is shown as more swollen and less 

 pointed. Schewiakoff in his excellent monograph of the Holotricha 

 ('96) says that in P. aurelia the spindle shaped body is equally 

 narrowed toward both rounded ends, whereas in P. caudatum 

 the anterior part of the body is but slightly narrowed, being almost 

 cylindrical in form, while the posterior part is much narrowed, so 

 as to be conical in form, though rounded at the tip. Moreover, 

 P. caudatum is said to have at the posterior end a bundle of longer 

 cilia, which is lacking in P. aurelia. These differences are clearly 

 shown in Schewiakoff's figures. Let us examine our various races 

 with these and simi'ar points in mind. 



Form. — It is difficult to make any precise statements that will 

 hold generally with regard to characteristic differences in form 

 between the caudatum and aurelia groups. It is not difficult to 

 find specimens of caudatum with the body broad in front and 

 drawn to a long point behind (for example, fig. 15, / or h), as de- 

 scribed by Ehrenberg and Schewiakoff; nor of aurelia with the 

 spindle-like form described by Schewiakoff (for example, fig. 17, 

 a or c^). But it is equally easy to find specimens of aurelia with 

 the supposed caudatum form (as fig. 15, e, fig. 17, 6), and speci- 

 mens of caudatum with the supposed aurelia form (as fig. 15, i). 



