140 M. straus-durckheim's 



owing principally to the absence of the feet. The nervous 

 system is variously influenced by this change in the segments; 

 and we may distinguish two forms of animals in this condition : 

 first, those where the mass of the viscera is contained in the 

 trunk ; secondly, those where it is contained in the abdomen. 

 In the former, the nervous system obeys the following laws. 



Second Law. — Where the trunk is composed of segments, 

 either moveable, immoveable, or anchylosed, but distinct in 

 the sternal region, whilst the abdomen is formed of perfectly 

 moveable segments; the pairs oi ganglia are repeated in each 

 segment of each part, their size being proportioned to that of 

 the organs of animal life contained in each segment ; and the 

 length of the chords of the spinal marrow is subject to the same 

 conditions as in the preceding law. 



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

 either moveable, immoveable, or anchylosed, but distinct in 

 the sternal part, whilst the segments of the abdomen are im- 

 moveable, and, whether anchylosed or not inferiorly, without 

 muscles to move them ; the ganglia are repeated, only in 

 the segments of the trunk, in the manner of the former case; 

 but the abdomen contains none, and receives its nerves from 

 the last pair of ganglia of the trunk, which is then larger 

 than the others. The terminal part of the chords of the spinal 

 marrow is prolonged nearly to the extremity of the abdomen, 

 where they distribute themselves. 



Fourth Law. — Where the trunk is composed of several 

 segments entirely united into one, or anchylosed so completely 

 that we cannot perceive any traces of the sutures of the dif- 

 ferent sternal pieces (the feet then radiating round a common 

 sternum), and the abdomen also is formed of segments entirely 

 anchylosed, be they otherwise distinct or not; we find only one 

 pair of ganglia furnishing all the nerves of this part of the 

 body. This pair of ganglia is placed at the centre from 

 which the feet radiate, (it is formed by the union of all the 

 ganglia of the segments which compose the trunk). \n the 

 abdomen there is no ganglion, (this part containing only vital 

 organs,) and the nerves ai'ise either from the ganglion of the 

 trunk, or from the chords of the spinal marrow, which are 

 prolonged to the extremity of the abdomen; but when this 

 part contains mixed muscles, (serving for respiration,) the 

 chords present a few extremely small ganglia. 



