CONSIDERATIONS. 141 



Where the mass of the viscera is contained in the abdomen, 

 the nervous system follows the subsequent laws. 



Fifth Law. — When the trunk is composed of segments, 

 either moveable, immoveable, or anchylosed, but distinct in 

 the sternal region, whilst those which form the abdomen are 

 perfectly moveable ; the ganglia are repeated in each part, but 

 with this difference, that those of the trunk are always very 

 large, and each segment has its own peculiar pair, whilst in 

 the abdo?nen they are much smaller, often less numerous 

 than the segments, and their situation is not always con- 

 stant. 



Sixth Law. — If the trunk is composed of distinct seg- 

 ments, whether these be moveable or anchylosed, and the 

 segments of the abdomen are very little moveable, or anchy- 

 losed, even if this is the case only in their inferior arches, the 

 ganglia are repeated in the trunk, as in the preceding case, 

 but not in the abdomen ; and the segments of this latter receive 

 their nerves from a large pair of ganglia placed in the anterior 

 part of the visceral cavity, or in the trunk itself. The chords 

 of the spinal marrow are prolonged nearly to the extremity of 

 the abdomen. 



Seventh Law. — When, on the one hand, the segments of 

 the trunk are entirely confounded, so as to leave no trace of 

 suture, especially on their lower part, (the feet then radiating 

 round a common sternum,) and, on the other hand, the seg- 

 ments of the abdomen are immoveable, whether confounded in 

 one or not, there exists in the trunk only one pair of ganglia, 

 as in the species which come under the fourth law ; and in the 

 abdomen there is but one single pair of ganglia, as in the 

 species which come under the sixth law. 



Eighth Law. — The brain, which exists in all the articu- 

 lated animals, is always placed above the alimentary canal, and 

 its size varies according to the number and nature of. the 

 organs to which it furnishes nerves. 



The Encephalo7i being found where the head has entirely 

 disappeared, seems to indicate that it is not subject to the 

 same changes as the latter. 



As it may happen, that in two neighbouring genera the 

 segments of the abdomen are moveable in the one, and fixed 

 in the other ; according to the second, third, fifth, and sixth 

 laws, the nervous system of these two genera ought to differ 



