NEMERTEA 121 



smaller, lies dorsal to the proboscis sheath, the other between 

 the proboscis sheath and the oesophagus. It will thus be 

 seen that the proboscis is surrounded by a nerve ring, a re- 

 lationship to the nervous system which is usually confined to 

 the alimentary canal of Invertebrates. 



The dorsal and ventral ganglion are separated by a deep 

 groove. The dorsal half gives off nerves to the eyes and 

 fore part of the head. The ventral half is continuous with 

 the main lateral nerve trunks. These two trunks run back on 

 each side of the body, embedded in the layer of longitudinal 

 muscle fibres. In ScMzonemertea there is a third lobe borne 

 on the dorsal aspect of the brain ; and in Haplonemertea this 

 lobe may be distinct and connected with the brain only by 

 a nerve. In some species it is hollow, and its walls ciliated. 

 In the last-named subdivision the longitudinal nerves give 

 off numerous segmentally-arranged nerves, but in the Palaeo- 

 and Schizo-nemertines these are replaced by a delicate plexus 

 which lies between the external longitudinal and the circular 

 muscles (Fig. 80). The main trunks may unite above the 

 anus in the Hoplonemertines, as in Peripatus and Gliaeto- 

 derma. A median nerve runs back from the supra-proboscidian 

 commissure and supplies the proboscis sheath and proboscis. 



The sense organs in Tetrastemma consist of four eyes 

 which seem to be little more than pigment spots devoid of 

 lens or other accessories. 



A ciliated groove exists on each side of the head ; each of 

 these leads into an oval sac which comes into close relationship 

 with the cerebral ganglia. These are the lateral organs, and 

 their nature is the cause of much discussion. They appear to 

 arise in the Schizo- and Hoplo-nemertines partly from the 

 epiblast of the skin and partly from the oesophagus. In the 

 Schizo-nemertines, where the nervous system is coloured red 

 with haemoglobin, they have been regarded as respiratory 

 organs ; but this does not explain their use in the other two 

 subdivisions, and they have been variously regarded as sense 

 pits and as excretory organs. The arrangement of their ex- 

 ternal ciliated openings affords a useful basis for classification. 



Tetrastemma, like most members of the class Nemeetea, is 

 dioecious. The ovaries and testes are arranged along each 



