12 
AUSTRALASIAN ANTARCTIC EXPEDITION. 
So far, then, as external features are concerned, we have to decide 
whether more importance is to be placed on proportions of body or on the 
form and size of papillee. As has already been pointed out, the former 
necessarily vary according to the degree of contraction of the muscles of the 
body wall, which may perhaps be so great as to discount the value of these 
proportional measurements. 
What other characters, then, are of use in distinguishing species \ 
The shape, and especially the dimensions, of the nephridia seem to be useless; 
they, too, are highly contractile, and their shape and length may vary on the 
two sides of the same worm. The number of intestinal coils varies apparently 
with the size; that is, with the age of the individual. I have noted that in 
three specimens differing in length the number of coils is proportional to that 
length, or rather, is less in the smaller individuals than in the larger. 
Lanchester found similar differences in his specimens of P. sociiim. 
There remains, then, the position of the origin of the retractor muscles 
of the introvert in relation to the anus or to the nephridiopores. For their 
distance from the hinder end of the body is more likely to be affected by the 
contraction of this region of the body and to a greater degree than the shorter 
distance from the anus. 
In the present specimens the ventral retractors arise at or behind the 
middle of the body length; the origin of the dorsal retractors is within the 
first quarter of that length. 
In order to be able to compare the positions of these points in the 
different varieties above enumerated, it is necessary to reduce the body length 
to a common unit, say 100, and to state the position of these origins in 
percentages of that length. 
The only tabulated series of measurements that I have met with is 
that given by Lanchester for P. sociwn ; he takes the distance from the 
nephridiopores, and it is necessary to deduct from his numbers the distance of 
these from the anus in order to bring them into line with the measurements 
given above. 
I have excluded from my comparison the specimens marked by him 
E and F, which differ in other features from the rest of his specimens and 
about which it is evident he was in some doubt as to their identity. I have 
reduced Lanchester’s numbers to percentages of body length, and plotted 
them on paper, with the result that the dorsal retractors in all these specimens 
of P. socium originate within the anterior third of the body as they do in my 
specimens; while the ventral retractors arise in the middle third, but this 
