398 On Alternate Generation of Annelids, 



length of the tentacles of the head, the second eye can be 

 distinguished, (PI. IX., fig. 17). At this point the three ten- 

 tacles of the head and the dorsal cirrus have about the 

 same length. If we trace further the development in dif- 

 ferent individuals, we find that there is a marked difference 

 in the gi-owth of the tentacles in parent stocks which are 

 identical; and this difference in the growth of the tenta- 

 cles is a sexual difference, the tentacles of the males beine 

 developed in one way, and those of the females in an en- 

 tirely different manner. When the head of the young has 

 reached the state represented in PI. IX., fig. 15, if it is a 

 male we soon notice on the inner side of the lateral tenta- 

 cles {a', a") a slight swelling, (PL IX., fig. 19) ; the tentacle 

 soon divides at this point by sending out a short branch, 

 which grows larger and larger, (PL IX,, fig. 20) ; the tenta- 

 cles remain attached together at the base. In the devel- 

 opment of the same tentacles in the female, we have 

 simply a lengthening of the tentacles, and a tendency in 

 the three tentacles to become well separated. So that a 

 female would gradually pass from the state represented in 

 PL IX., fig. 17, to that of PL IX., fig. 18. The head becomes 

 more and more distinct ; the small, tentacular cirri (a'") (PL 

 IX., fig. 20) of the males become visible as a slight protuber- 

 ance on the lower surface, when the tentacular cirrus (c) 

 is about one half as long as the tentacle. At the same 

 time in the males the long dorsal cirrus (c') and the mid- 

 dle tentacle (a) assume the curled attitude in which they 

 are usually carried by them, while in the females these 

 tentacles simply increase in length, the tentacle (a) always 

 being carried in advance of the head, and not turned back 

 as in the males. The five rings following the head in the 

 males (six in the case of the females), undergo scarcely any 

 change, with the exception of a slight elongation of the 

 dorsal cirri, (compare PL IX., figs. 1 and 9 and PL XI , fig. 

 8) ; but in the succeeding rings very striking differences are 



