6 
CLASSIFICATION. 
by and owing to the development of the shell as a protection and 
covering to the vital organs of the body, and this shelly covering 
not having retained its equilibrium has fallen over to one side and 
Fig. 2. — Diagrams illustrating the progress and method of the assumed rotation of the 
internal organs in the Anisopleura (after Lankester). The arrows indicate the direction of the 
rotatory movement. A. nnrotated ancestral condition ; K. semi-rotation of the organs partly 
accomplished; C. complete semi-rotation.— < 2 . anus; r.n. the primarily left and primarily 
right nephridia or kidneys ; primarily left visceral ganglion, which after the torsion is 
accomplished forms the sub-intestinal ganglion ; r.^-. primarily right visceral ganglion, which 
subsequently becomes the supra intestinal ganglion. 
to the i-eav, compressing and gradually displacing the termination of 
the intestine and causing it to assume the position it now occupies. 
The Isopleura, of which the C/tltoiis are examples, are practically 
organized in a bilaterally symmetrical manner externally and 
internally, the viscera not having been subjected to the torsion 
alluded to. The Anisopleura to which all our land and freshwater 
species belong, though presenting externally a bilaterally symmetrical 
ai)pearauce of the head and foot, have been subjected to changes in 
the position of the chief viscera by this semi-rotation of the visceral 
sac ; they may be sub-divided into two Orders called Eutiiyneura 
and Streptoneura, this separation being based upon the modi- 
fication caused in the arrangement of the visceral nerve-loop, 
by its being involved in, or escai)ing from, the twisting of the 
viscera. In the Eutiiyneura, of which Limnau stagnalh is an 
exanqile, the visceral nerve-loop is often comparatively short, and 
on account of lying beneath the intestinal canal has escaped the 
twisting to which the iqiper portion of the viscera has been sub- 
jected. The Streptoneura differ from the Euthyneures owing to 
the visceral nerve-loop lying above the intestines and being thus 
involved in the twisting and rotation the organs have undergone. 
