846 WINTERTON C. CURTIS 
to indicate anything except the ineffectiveness of the attempt at 
expurgation. There can be no interpretation of the facts which 
would show that the 8. polymorphus had given rise to these 
Crossobothria. 
Despite the lack of intervening stages between the S. poly- 
morphus and the specimens of P. triloculatum, which I found 
in these sharks, I think there is some pretty good evidence 
that the latter have developed from the introduced 8. polymor- 
phus. First, the examination by myself, and also by Linton, of 
a large number of sand sharks at various times has never shown 
that young or adult specimens of Phoreiobothrium triloculatum 
are to be found as regular parasites of this shark, or as frequent 
‘xenosites.’ I have never found this form in any sharks except 
those which I infected with the 8. polymorphus. It is very prob- 
able, however, that one might at any time find stray specimens of 
this worm, since the squeteague is a not uncommon food of this 
shark. The failure to find it in any of the sharks I have examined 
would indicate that it does not often survive, when introduced in 
nature along with the squeteague, and in my experiment only a 
small proportion of the larvae introduced have survived. Second, 
the specimens of P. triloculatum which I found in any one shark 
were of very uniform size, and unless we suppose that there is 
some limit to their growth, when in an abnormal environment 
(the wrong host), this would indicate that they all entered the 
shark at about the same time. In the third place this conclusion 
is also in line with the results of Monticelli (’88) who has shown 
that the S. polymorphus of European waters develops into the 
genus Calliobothrium, which belongs to the same family as Phore- 
iobothrium, a fact which is further discussed in the portion of 
this paper which deals with the nature of the 8. polymorphus. 
Attention should here be directed to one point which has per- 
haps suggested itself from the examination of the figs. 7-13. 
This is that the specimens of the young P. triloculatum (figs. 9 
and 10) are considerably smaller than some of the specimens of 
the S. polymorphus, from which I suppose them to have devel- 
oped. This appeared to me at first a most serious objection, 
though upon further consideration it does not seem an insur- 
