STOLON FORMATION IN SPECIES OF TRYPANOS YLLIS. 423 
the eye- and brain-bearing first segment ■which shows its 
primitive chai’acter by also carrying parapodia equipped with 
dorsal cirri, setae and acicula.” I have confirmed this in the 
examination of a large number of the stolons of the Victoria 
specimens. In tlie possessions of the typical “ tete acere ” 
T. ge mini par a resembles T. krohnii ( = T. zebra), and 
T. crosslandi is thus the only member of the genus which 
produces so advanced a type of stolonial head. 
A third point of difference presents itself in the development 
of the caudal cirri. In T. gem mi para they are the earliest 
and most prominent features, and are much longer, thicker, and 
consist of more annuli than the dorsal cirri of the posterior 
segments, in stolons of all stages of development. In T. 
crosslandi, on the other hand, they are so small as to be 
hardly noticeable. They are equalled in size by the dorsal 
cirri of the stolon, although these themselves are less developed 
than in T. gemmipara. 
It should be noticed that the setfe of the stolons of both 
species resemble each other in type. At first appearance the 
terminal piece, triangular in shape, appears to have a smooth 
internal margin, but closer examination shows a minute tooth 
just under the apex. This seems to suggest that T. cross- 
landi is derived from a species (like T. gemmipara) with 
such a seta in the adult. 
'1’he Internal Changes accompanying Stolon-Formation. 
The earliest stage of the phenomenon is shown by (1) of 
I', crosslandi, in which several rows of stolons have been 
marked out but none of the stolons have become seg- 
mented. This was examined by means of a series of ver- 
tical longitudinal sections, and typical members of these are 
drawn in Text-fig.s. 1 and 2. It is here shown that prolifera- 
lion has given rise to a cushion of undifferentiated tissue 
standing out from the ventral surface of the stock. It is 
lormed by ectoderm and mesoderm alone, and the former is 
indented by furrows which mark off the incipient stolons. 
