152 COMPARATIVE ANATOMY. 



is composed of four lobes, gives off four nerve-trunks, which 

 pass backwards. Two of them have a central, and two a lateral 

 course. They unite in a ganglion near the end of the body. In 

 Neomenia there is a considerable complication. The cerebrum gives 

 off a commissure, which surrounds the oesophagus, and of these also 

 a commissural chord on either side, each of which passes to a ganglion 

 at the side of the oesophagus ; from each ganglia a lateral nerve-trunk 

 is given off. The lateral nerves unite in a terminal ganglion 

 (branchial ganglion). A commissure passes from each of the 

 lateral ganglia to a ventral ganglion, which gives off a ventral nerve- 

 trunk, which is connected with its fellow of the opposite side by a 

 number of transverse commissures. If the lateral and inferior pair of 

 ganglia be regarded as parts separated off from the cerebrum, this 

 form would be seen to approximate very closely to Chastoderma, 

 and the only difference would lie in the oesophageal commissure, 

 in the transverse commissures of the ventral trunks, and in the 

 exclusion of the latter from any share in the terminal ganglion. 

 In any case we have in Neomenia a further development of the 

 simple characters of Chsetoderma. This is not the place to indicate 

 further points of comparison, for as yet we are only beginning to 

 obtain any exact knowledge as to the structure of these animals. 



Sensory Organs. 



Tactile Organs. 

 § 122. 



The sensory organs of the Vermes are of a high grade of 

 differentiation. The organs of tactile sensation appear in 

 the form of fine modifications in the structure of the integument 

 with which the peripheral nervous system enters into connection. Of 

 this kind are the true tactile organs, while the coarser arrangements, 

 such as the processes of the integument, are only bearers of them. 

 The essential part of these organs consists in the connection between 

 the sensitive nerve-fibres and the modified cells of the integument ; 

 these cells, as a rule, project beyond the surface of the integument, 

 as stiff setiform processes (tactile setre, or rods). These arrange- 

 ments are most exactly known in the Rotatoria and Annelida, but 

 they have been recognised in other divisions. 



Tactile setae are widely distributed among the Turbellaria and 

 Nemertina, where they are sometimes found over the whole body, 

 and sometimes are richly developed on the head. They are found on 

 the tentacles of the Bryozoa; and arc widely distributed on the 

 cephalic segment in the Lumbricida), and in the Chastopoda. They 

 appear in the Cha)topoda on the true tentacles and antenna?, as also 



