NEMER TINES. 259 



between the retractile proboscis of the Nemertines and 

 the non-retractile proboscis of Balanoglossus (Bateson). 



In the most primitive Nemertines the nervous system 

 consists essentially of a somewhat complicated pair of 

 cerebral ganglia and a diffuse nerve-plexus, with nerve- 

 cords lying at the base of the ectoderm.* As the cerebral 

 ganglia probably belong to the same category as the cere- 

 bral ganglia of all other typical Invertebrates, and are not 

 represented in Balanoglossus, we can afford to neglect 

 them at present. Confining our attention to the ecto- 

 dermal nerve-plexus, we find occurring in it, along definite 

 lines, local thickenings, after the same principle, but not 

 all on the same lines, as was described above for Balano- 

 glossus. Directly comparable with the dorsal longitudinal 

 nerve-cord of Balanoglossus, there is a similar thickening 

 or concentration of the integumentary nerve-plexus in 

 some of the Nemertines, in the dorsal middle line {Car- 

 inina, Cephalotln'ix). Hubrecht, who discovered this, calls 

 it the medullary nerve. There is, however, no correspond- 

 ing ventral nerve-cord in the Nemertines, but, instead of 

 this, there is a pair of lateral thickenings, constituting the 

 well-known lateral nerves of the Nemertines (Fig. 124), 



It is usually supposed that the lateral nerves of the 

 Nemertines are homologous with the two halves of the ven- 

 tral nerve-cord in the Annelids. In the Annelids the 

 primitive lateral nerves (which are so typical of the Platy- 

 helminths, or flat-worms) have approached one another in 

 the mid-ventral line, and have often undergone intimate 

 fusion together. In some cases, however, they are separated 

 from one another by a wide interval (Sabellaria, etc.). 



* Hubrecht compared the lobes of the cerebral ganglia of a Nemertine to 

 the cranial ganglia of the Vertebrates, the lateral nerves to the Rami laterales 

 vagi, and the proboscis-sheath to the notochord. 



