ca) 
50 
ing exact measurements of these structures must be duly regarded. Exact 
measurements are possible only in cases where the hooks are viewed in 
full profile; accordingly, in a permanent mount usually not more than one 
or two hooks of any given circle will afford conditions for obtaining an 
accurate determination of the maximum length of the hooks. For all 
other hooks there is usually a foreshortening produced by the angle at 
which the hooks protrude toward the observer from the proboscis-wall. 
In order to determine the extent to which variability in hook-length may 
go in N.agilis, the writer has taken numerous measurements in which 
especial care was exercised to eliminate inexact and incorrect observa- 
tions. In Table V are brought together a few typical examples of meas- 
urements which were obtained from a study of individuals from the 
collections of Parona and of Monticelli. 
TABLE V 
VARIABILITY OF HOOK-LENGTH IN N. agilis 
t (Measurements are in ») 
: g Terminal hooks, Middle hooks, Basal hooks, 
Slide No. va length length length 
Parona Coll. 
618 fe) 94 —* 41-47 
619 4 120 val 59 
620 ) 120 71 59-65 
622 é 120 77 71 
623 3 120 65 65 
624 a 120 83 59-71 
Monticelli Coll. : 
714 | é 94-100 53 35 
715 sense 100 53 30 
716 Reade 94-100 41-47 - 80-35 
717 fe) tips broken 47 35 
718 fe) tips broken 47-53 35 
719 | é 106 71-82 53-65 
720 | é 106 65-71 47 
* No hook in a position for an accurate measurement. 
Because of the differences in data concerning N. agilis presented by 
various workers, only a part of which seem explicable on the ground of 
individual variability within the species, it seems desirable that a full 
statement be given covering the diagnostic characteristics of this species. 
The following description is based upon the comparison of recent Euro- 
pean collections with Rudolphi’s types of the species. In the case of vary- 
ing structures an attempt has been made to indicate the usual range of 
such variability. 
NEOECHINORHYNCHUs AGrILts (Rudolphi, 1819) 
(Pl. XXVIII, Fig. 36-44) 
Specific Diagnosis—Maximum diameter in anterior third or fourth 
of body. From this region body tapers gradually toward each extremity. 
