THE ENTEROPNEUSTA. 665 
There is more irregularity here in the figures for the genito-hepatic length and_ total 
length, which perhaps is only to be expected in view of the smaller number of specimens. 
There is however no uniformity indicating a growth change. The value for group 3 in the 
branchial series is far larger than for groups 1 and 2. This however is due to the large 
branchial region of a single specimen (Table 13, No. 49) and its effect on the few individuals 
of group 3. The closeness of the figures for groups 1 and 2 (1°65 to 1:62), where there 
are more specimens available, points to the ratio of branchial length to collar not being 
affected by growth changes. Assuming then, and the assumption seems a fair one, that 
growth has little or no effect on the relative proportions of the body in Ptychodera we have 
before us an easily applied criterion for distinguishing different positions of organic stability 
in this genus. We have but to determine one of these ratios (and in practice the branchial 
one will be found the most convenient) on a number of specimens of each supposed variety, 
and to apply to the series so obtained the ordinary biometric methods for measuring the 
variability. Marked differences occurring in two groups of specimens would at once cast a 
doubt on the identity of two such groups. 
This method has been adopted here for such of the varieties as possessed nine or more 
specimens. The branchial ratio alone was used owing to the fragmentary condition of most 
of the individuals in each variety. On working out the mean and standard deviation in 
each case the following results were obtained :— 







TABLE 8.’ 
Pt. laccadivensis | Pt. maldivensis | Pt. muscula Pt. gracilis Pt. parva | 
| (Minikoi) C4 (Goidu) (Hulule) (Turadu) 
a —— — —| es = = — 
M. 2263 1-630 | 2°225 _ 1259 1514 
P. E. M. + 1405 + 1724 } + “2484 + +1995 1753 
o + 2°3098 | +1:8431 | +1:1647 . +1°5656 7798 
P. E.o + 0993 | + 1219 iO es Neti “1240 
C.V. 102°07 | 113°08 52°35 124°35 51°50 
N. 123 52 | 10 28 | 9 


These results shew clearly that in certain cases we are without doubt dealing with different 
races’ of Pt. flava. Thus the two forms laccudivensis and maldivensis are exceedingly alike 
both in external appearance and in internal structure. The branchial ratio is however 
very different, the mean for maldivensis (1°63) being very much lower than that (2°26) for 
laccadivensis. And the difference between the two is almost four times the probable error of 
either, from which it may be calculated that the odds are many thousands to one against 
its being due to errors of random sampling alone. 
! For the actual measurements on which these figures are 
based see Tables 11—13, pp. 671—4. Of the letters in the 
(ef. Davenport, C., Statistical Methods, 1899, p. 38). My 
point is that in the group of animals collectively designated 
above Table (i.e. Table 8), M.=mean; P.E.M.=probable 
error of the mean; o=standard deviation; P.E.c.=prob- 
able error of standard deviation; C.V.=coefficient of varia- 
tion; whilst N.=number of specimens in each case. 
*I am not here concerned with the question of the 
statistical method as a criterion of species and variety 
Pt. flava there are to be found different positions of organic 
stability, and in giving to these ‘‘a local habitation and a 
name”’ there is no thought of distinguishing by statistical 
methods between the terms, race, variety, and species. For 
the value attached to such distinctions must for the present 
be largely a matter of individual taste. 
