146 COMPARATIVE ANATOMY. 



longitudinal nerve-trunks (n) which pass out from them, form the 

 principal portion of the nervous system ; from it finer branches pass 

 off to the sensory organs of the integument {u '), to the dermo- 

 muscular tube, and to the internal organs. The longitudinal trunks 

 pass along the lateral edges of the body, and are placed closer 

 together, or are more widely separated from one another, according 

 to the breadth of the body. In the dendrocoelous Turbellaria, as well 

 as in many Trematoda, these lateral longitudinal trunks are only 

 shghtly developed, so that it is difficult to separate them from the 

 other nerves, which arise from the cerebral ganglia, although they 

 are not unfrequently distinguished from the other nerves by their 

 larger size. 



The Rotatoria come nearest to the Platyhelminthes. The central 

 organ is a ganglionic mass lying on, but never surrounding, the 

 oesophagus. In some it is distinctly separated into two lateral 

 halves. The peripheral nerves arise from this cerebrum; and as 

 they are not collected into longitudinal trunks, the simplest form, 

 which is most like to that of the Turbellaria, obtains in this group. 



. The nervous system of Pedicellina appears to be of this low grade, 

 for it is placed on the stomach, and does not form an oesophageal 

 ring. It is not quite certain whether or no the ganglionic masses, 

 which lie on the oesophagus in Echinoderes, are separated from one 

 another. If there is a dorsal commissure the arrangement would 

 be similar to that in the lower Platyhelminthes. 



The nervous system of the Bryozoa is more highly developed ; 

 its single central mass is a simple ganglionic swelling, lying between 

 the mouth and anus, and sending out, in addition to large branches 

 for the tentacles, two nerves which form a ring around the com- 

 mencement of the oesophagus. Where the nervous system is most 

 exactly known, as in Alcyonella, there is no doubt about the 

 oesophageal ring. Prom the lateral part of the central nerve-mass 

 a lobate process goes to the lophophore, and, like the rest of the 

 oesophageal ring, gives off nerves to the tentacles. 



In addition to this nervous system in each individual, a colonial 

 nervous system has been recognised in the stock, but it is not 

 quite certain that this system does exist. 



§ IIG. 



The nervous system of the Nemathelminthes appears to be ar* 

 ranged in a special manner, so far as the facts about it are agreed 

 upon. It consists of a central organ placed on the oesophagus, and 

 surrounding it as a ring, from which nerves radiate forwards, as well 

 as backwards. This distribution of the nerves corresponds to the 

 arrangement of the ganglionic cells of the oesophageal ring. The 

 nerves which run forward from it may be separated into six iibrous 

 bundles. Two run in the middle of the lateral tracts, and four in 

 the direction of the secondary median lines. Ganglionic cells lie 

 both on their origin and their course. The nerves which pass back- 



