MONSTROSITIES — SINISTRORSITY, ETC. 109 
The causes of this reversal of the normal arrangement are however 
not at all known or understood. M. Bourguignat has hazarded the 
suggestion that it may be caused by electrical conditions, the electric 
current flowing in the opposite direction to the embryonal rotation, 
the essential conditions being a metalliferous soil, moist weather to 
influence the latent electricity of the metallic substances, and the 
conjunction of the atmospheric and terrestrial electricity, as by 
thunder at the period of the first manifestation of vitality l)y the 
embryo. Prof Cams also considers that the direction of the coiling 
of the shell and animal may possibly be determined by the direction 
of the embryonal rotation. 
Mr. 11. Ellsworth Call attributes sinistrorsity in Mehintho to the 
crowding of the embryos in the oviduct in the early stages of their 
existence ; his observations on that genus lead to the inference that 
sinistral specimens are more delicately constituted than their 
normally coiled brethren, as he found that sinistral exami)les con- 
stituted li- per cent, of the total number of the embryos in the 
oviduct of Mehmtho integra ; and 2h per cent, in Mehintho decha, 
while judging from the numbers gathered in the adult stage, he 
found only one-tenth per cent, survived. 
In Limnwa Mr. II. E. Crampton, jun., has recorded that the ilirec- 
tion of the cleavage during segmentation of the ovum, is of the 
typically spii’al type, but in Physri the direction of this cleavage is 
totally reversed. Dextrorsity and sinistrorsity are however (pialitios 
inherent to the organization of the animal, and the particular arrange- 
ment is usually correctly indicated by the position of the heart and the 
external orifices of the reproductive and other organs of the bod^o 
All embryonal Castropods are 
primitively what are termed 
Exogastric, and oidy become 
Endogastric owing to the tor- 
sion of 180° which is under- 
gone by the visceral ,sac, which 
twisting transfers the anus and 
other organic orifices from the 
rear to the anterif)r j)art of the 
animal ; the incii)lent spire at 
first enrolled towards the front or dorsally with reference to the 
later whorls, becoming coiled towards the rear. According to Prof. 
Fig. 234. Fig. 13,5. 
Exogastric coiling. Endogastric coiling. 
Diagrams showing the process hy which an 
Exogastric mollusk becomes Endogastric by the 
torsion of the visceral sac through an arc of 180 , 
a process determining the sinistrorsity or dextror- 
sity of the shell (after Pelseneer). a. anus. 
(The figures, for the sake of clearness and in- 
telligibility, are drawn to represent a much later 
stage of embryonal development than that at 
which the torsion actually occurs.) 
