968 THE AMERICAN NATURALIST. 
fourtypes. The abnormal multiplication of claws in Pectinatella 
is paralleled by the normal condition in Cristatella, where the 
radial outgrowths of the statoblasts form grapnels with many 
claws. The condition in Cristatella is in so far different as the 
claws stand out in all radii in Cristatella, whereas they lie almost 
or wholly in one plane in the case of Pectinatella. Also 
* Pectinatella Carteri” has ** many lateral claws " to each hook. 
Both abnormalities and individual deviations from mediocrity 
in Pectinatella are thus normal or modal in other species of 
Phylactolzemata. 
Summary. — The hooks on 827 statoblasts of a Lake Michi- 
gan Pectinatella were counted. The number ranged from ri 
to 21 ; the mode was 13; average, 13.782 + .031 ; and index 
of variability, 1.318 +.022. The skewness was + 0.077 ; that is, 
the variation tends in the direction of the larger numbers, and 
this is correlated with the fact that species or races of Pectina- 
tella with much larger numbers of hooks occur. There is a 
slight hereditary tendency in the statoblasts from one colony, 
about 0.1 when 1 is the maximum inheritance. There is an 
inverse correlation (of — .092 + .006) between the number of 
hooks and the perimeter, and a larger one between number and 
size of hooks. The number of hooks is thus not determined | 
by room, nor does it seem to be predetermined from an early 
stage of development of the statoblast. The hooks show abnor- 
malities, some of which resemble the normal condition of hooks 
in Cristatella. 
LITERATURE CITED. 
'56 CARTER, H. J. Ann. and Mag. Nat. Hist. III. 
'66—68 Hyatt, A. Observations on Polyzoa, Suborder Phylactolemata. 
Proc. Essex Inst. Vols. iv and v ; also separate. 103 pp., 9 pls. 
'90 OKA, A. Observations on Fresh-Water Polyzoa. Jour. Coll. Sci., 
Imper. Univ. Japan. Vol. iv, pp. 89-150, Pls. XVII-XIX. 
'95 PEARSON, K. Contributions to the Mathematical Theory of Evolu- 
tion. II. Skew Variation in Homogeneous Material. PAz/. Trans. 
Roy. Soc. London. Vol. clxxxvi, A, pp. 343-414, 10 pls. 
'98 PEARSON, K., AND FILON, L. N. G. Mathematical Contributions, 
etc., iv. On the Probable Errors of Frequency Constants, etc. 
Phil. Trans. Roy. Soc. Vol. cxci, A, pp. 229-311. 
