TORSION OF THE BODY. 
208 
tlie space at the anterior right side of the pallia! cavity which the 
atrophied organs wonld normally have occupied ; the organs of the 
left side of the cavity have, however, moved backwards towards the 
right and filled the vacant space ; hy this nnwinding movement or 
l)raetical partial detorsion the aims, which was placed near the median 
line aliove the neck, eventually came to occupy a position at the 
extreme right of the mantle cavit}". 
In the Streptonenres this torsion of the body is gTeater than in the 
other gTonps, but the asymmetry of the internal organs is not always so 
complete as in the less distinctly twisted Euthynenres, some genera 
still retaining the primitively paired gills, nephridia, and other organs, 
hnt the i)allial opening is placed at the front, over the neck of the 
animal, and the pleuro-ahdominal commissures are crossed, both cir- 
cnmstances owing to the twisting undergone hy the body. 
In the Euthynenres, although the twisting of the body is less, as 
shown liy the lateral opening of the pallial cavity and the relative 
positions of the pallial organs, yet the asymmetry of the internal 
organs is greater, as all traces of the originally left half of the primi- 
tively j)aired organs have disappeared and renders reasonable the 
sn})position of Prof. Pelseneer that the Euthynenres are to he regarded 
as derived from and as in a more advanced stage of specialization than 
the Streiitoneures, which are our most primitive forms and that the 
less amount of body twist and greater internal asymmetry they exhibit 
is owing to a process of partial detorsion which the Euthynenres have 
j)rohahly undergone ; all those animals whose visceral dome has under- 
gone com[ilete detorsion, as shown Ijy the return of the respiratory 
and other organs to their primitive position at the rear of the animal, 
are liable to the disappearance or partial atrophy of the mantle and 
shell, as strikingly shown in the Oinsthohranchs and in the terrestrial 
genus TcstaceUa, which is distinctly opisthopneumonic, the respira- 
tory organs being now posterior to the heart. 
Although our Pelecypods show little sign of having undergone any 
noticeable torsion of the body, except such coiling as the Prosogyrate 
umhones indicate, yet the group is not exempt from it, as the animals 
of the genus Trlducna clearly show that their body in its relation to 
the shell has been snbjected to an anterior rotation or twist of 
nearly 180°, which has brought the posterior adductor muscle to 
near the ventral margin of the shell and the mouth or oral aperture 
close to the umhones. 
