MONILIGASTER GRANDIS, A. G. B. 357 
this modification obtains only when the worm is fully adult ; 
in other species even in the adult this modification does not 
obtain. I express these two conditions by saying “ ovaries 
enclosed ” or ‘ ovaries free” as the case may be. 
Muscles.—The longitudinal muscles have in some and not 
in other species the “ feathered ” arrangement. 
Setal muscle-bands are present or absent according as the 
setz project or not into the celom. 
Alimentary Canal.—The number of the gizzards and the 
particular segments in which they occur, although subject to a 
considerable amount of individual variation, is to a certain 
extent a good specific character. It is not difficult by examin- 
ing a number of individuals to determine the number of 
gizzards normal for the species, and it is possible, by examin- 
ing a still larger number, in some cases to fix upon a normal 
position. It is very common to find a slight gizzard develop- 
ment in front of or behind the normal gizzards, and sometimes 
these additional gizzards are as large as the others. The giz- 
zards which do develop are always in contiguous segments. 
There are rarely if ever more than one or two gizzards in 
excess of the normal number. The variation in position de- 
pends of course for one thing upon the number of gizzards 
developed, but given the normal number the position may still 
vary, especially in some species, by two or even three seg- 
ments. It is evident that the cesophageal wall in the gizzard 
neighbourhood can very easily produce these muscular deve- 
lopments, and the variation in position is fully accounted for 
by the fact that the mesenteron is not primarily metameric- 
ally segmented, and that the portion of the gut bounded by any 
particular septa is not necessarily homogenous with the portion 
bounded by the homogenous septa, in another individual. 
A typhlosole is never present. 
Vascular System.—This presents slight individual varia- 
tions only, but certain well-marked specific variations, some of 
which I describe elsewhere in this paper. 
Nephridia.—I have not specially studied the variations in 
the structure of the nephridium ; so far as my observations 
