182 



THE NEMERTINI 



with it (Fig. XIX.). The two ganglia are developed from different 

 "rudiments," the dorsal possibly represents the prostomial "cere- 

 bral ganglia " of the Annelids ; the ventral probably represents the 

 suboesophageal ganglion of an Annelid. But here in the Nemer- 

 tines there is no repetition of ganglia ; these two ganglia constitute 



the " brain," and are so closely 

 united in the lower forms that 

 in Hubrechtia there is no ex- 

 ternal demarcation between 

 them. But usually they are 

 distinct as a dorsal and ventral 

 lobe of the brain. The dorsal 

 lobe is usually the larger, and 

 is connected with its fellow by 

 a delicate, supra - proboscidial 

 commissure; the ventral lobes 

 are connected by a broader 

 commissure below the proboscis 

 tube. The "cerebral organ" 



Brain of Eupolia giardii, Hubr. (from Perrier, frequently becomes Very closely 



associated with the hinder part 

 of the dorsal ganglion (Fig. 

 XIX. P), or a special ganglion 

 may separate from it, to be- 

 come connected with the organ (Fig. XX. L). The dorsal lobe 

 is essentially sensory ; the ventral motor. 



The third longitudinal cord is thin, and arises from the supra- 

 proboscidial commissure; it always retains its superficial sub- 

 epidermic position, even when the rest of the system has sunk 

 into the parenchyma (Metanemertines). In all but these it gives 

 off a branch which passes below the circular muscles, and runs 

 back as a second dorsal nerve (see Figs. VIII., X. Ic). Hubrecht, its 

 discoverer, called it the " proboscidial-sheath nerve." l These three 

 longitudinal nerves specialisations as they are of a primitive net- 

 work of cells and fibres are connected by this network, or tunic, 

 in the Protonemertini and Heteronemertini, in some of which, 

 especially Hubrechtia, it attains a considerable thickness. Even in 

 Carinella this primitive nerve plexus exhibits a tendency to form 

 circular, commissural nerves, for the circular strands are more pro- 

 nounced than the rest. In the Metanemertines this nerve tunic has 

 become specialised, in connection no doubt with the sinking of the 

 whole system, for it is represented by a ladder-like series of ventral 

 commissures connecting the lateral stems (Fig. XX.), and by a series 



1 For a suggestion as to the importance of the dorsal nerve, as well as of the 

 proboscis and its sheath of Nemertines, in the evolution of Vertebrata, see Hubrecht 

 (18). 



FIG. XIX. 



after Hubrecht). c, aperture of cerebral organ 

 cd, dorsal commissure ; cv, ventral commissure ; 

 L, lateral nerve trunk ; M, middle lobe of the 

 dorsal ganglion ; P, posterior lobe (cerebral 

 organ) ; S, dorsal ganglion. 



