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



In I (aurelia) the posterior part of the body hardly tapers back- 

 ward at all, and the posterior end is smoothly rounded, whereas 

 in Lo (caudatum), the posterior region tapers sharply backward 

 and the posterior point persists even when the body becomes very 

 thick; it stands out as a marked protuberance. In no case was 

 such a protuberance present in the specimens of aurelia. Besides 

 this difference, the specimens of the caudatum race were more 

 slender than those of the aurelia race. In the former the breadth 

 was 34.1 per cent of the length; in the latter 38.1 per cent. 



No characteristic differences in form have been noticed between 

 the races of different size belonging to the same group or species 

 (caudatum or aurelia), though much attention has been directed 

 to this point. A study of characteristic forms of the large aurelia 

 race k and of the small aurelia race i, under identical conditions, 

 is given in figs. 21 and 22. 



Within any single race the form of the anterior end differs, 

 depending on whether one sees it from the side, or dorso-ventrally. 

 As seen from the side the anterior end is narrow and pointed (as 

 in Schewiakoff 's figure of P. aurelia) ; in dorso-ventral view it is 

 broad and blunt (as in Schewiakoff 's figure of P. caudatum). 

 These differences are well brought out in the two views of a single 

 individual of k (fig. 17, a and c), of i (fig. 17, g and h); they are 

 well seen in Lo by comparing h and i, fig. 15. The differences 

 between dorso-ventral and lateral views become much less marked 

 when the animals are very plump, as in fig. 20. 



The posterior part of the body is usually circular in section, 

 but when the animals are starved, they become greatly flattened 

 dorso-ventrally, as illustrated in the two views of an individual of 

 the race c given in fig. 23. Often under these conditions the ani- 

 mals become ridged and folded, so as to be quite irregular in sec- 

 tion. 



Posterior tuft of long cilia. — This has commonly been set forth 

 as a distinctive character of caudatum, but if there is any 

 distinction in this respect, the difference is extremely difficult to 

 detect. No matter how carefully killed, a large proportion of the 

 individuals in any race will not show this posterior tuft of long 

 cilia, owing doubtless to the irregular and entangled positions of 



