and the Embryology of Autolytus cornutus. 391 



cirri of the rings with long setae are exceedingly short 

 when compared with those of the anterior rings. It dillers 

 from S. Hclgolandica in having a larger number of rings; 

 the rings without the long setse are more numerous in both 

 the sexes than in Ilelgolandica. In the latter the female 

 has two, and the male three rings, having only stiff bristles, 

 while in the cornutus the female has six, and the male five 

 rings, in which the long setsB are wanting. The number of 

 rings of Autolytus cornutus in the full-grown males and 

 females is different : usually from twenty-seven to thirty in 

 the former, and from thirty-five to forty or even fifty in the 

 latter. Sacconereis Helgolandica has fewer rings, and P. 

 longosetosus a greater number than the species found on 

 our coast. 



The most striking characteristic of the genus Autolytus 

 is the remarkable difference between the sexes. Had I not 

 observed the development of these males and females, so 

 widely different, coming from parent stocks * (from which 

 they are produced by transverse division, as I shall show 

 hereafter), developed from eggs laid by the same individ- 

 ual, I could scarcely have credited their generic identity. 

 (See PL IX., fig. 1, and PI. XI. fig. 8.) It is therefore per- 

 fectly natural that Johannes MQller should not recognize 

 in a female Autolytus the genus Polybostrichvs, which was 

 characterized by Orsted from a male. At the time when 

 Max MUUer discovered his Sacconereis He/golandica, he 

 used to find also in great numbers a small worm (invaria- 

 bly a male), which he says had the general appearance of 

 Sacconereis^ but differed in such a remarkable manner, that 

 it seemed to him impossible that there should be such an 

 exceptional case of difference between the males and fe- 

 males, entirely unprecedented in the class of Worms, It 

 was only after tracing the complete development by 

 " transverse division " of males and females, differing in 



* See below for description of parent slock, p. 397. 



