278 
The dorsal vessel is markedly developed ; its contractility 
becomes specialised and confined in great measure to that 
part which overlies the so-called stomach. Its walls are 
thick in this part, and contain clear nodular masses, which 
may be nervous elements, or may act, as they appear to do, 
as valvular thickenings. They are not dissimilar in appear- 
ance to the masses seen in the dorsal vessel of some insects, 
e.g. the Corethra larvae, and may be compared also with 
the multicellular valvules mentioned by Leydig in his 
‘ Lehrbuch ’ as occurring in Clepsine, Piscicola, and 
Fontobdella. 
It will be observed, in the outline figure of the sexual Chceto- 
gaster Limnaii (PI. XIV, fig. 2), that the first pair of abdominal 
fasciculi is represented as overlying the stomach in its ante- 
rior half, whereas in the larvae (PI. XIV, fig. 3) the first pair 
always appear at the extreme posterior end of the stomach, or 
even altogether posterior to it. The range of movement of 
which the alimentary canal is capable, of course, affects the re- 
lations of these parts somewhat, but will hardly account for the 
exceedingly forward position of the first pair in this case. It 
Avould, no doubt, be interesting to ascertain how far the adult 
Chcetogaster is jn'oduced in the course of posterior growth, and 
how far its special characters are due to subsequent modifica- 
tion. Not having watched the mode of development of the 
additional bristles which are present in the sexual form, I am 
doubtful as to whether this first pair of fasciculi is really the 
representative of the first pair in the larval condition. It is 
probably a new development — the second pair of the sexual 
corresponding to the first pair of the larval form. The rela- 
tion to the opening of the first pair of segment organs should 
determine this point ; but my notes are not absolutely de- 
cisive on this relation. 
In the larval Chcetogaster Limned there is, as I have repeat- 
edly observed, although the specimens measure barely one tenth 
of an inch in length, a pair of segment organs situated in front 
of the first pair of fasciculi, and opening in front of them, 
there is also a similar pair in front of each succeeding pair of 
well-developed fasciculi, connected always more or less with a 
muscular diaphragm forming the anterior boundary of the 
segment to which the organ belongs. In Chcetogaster dia- 
phanus, whilst still in an asexual condition, there is no such 
large gap between the cephalic and first abdominal fasciculi 
as exists in the larval Ch. Limneei, nor is there, according to 
his figure, in the Ch. filiiformis 1 (with three bristles in a fas- 
1 Roughly figured — observed in the Andes ‘ Neue Wirbellose Thiere.’ 
The gap between the cephalic and succeeding fasciculus in Ch. Limneei is 
