182 



THE NEMERTINI 



cv 



cd 



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 - proboscid^l 

 commissure; the ventral lobes 

 are connected by a broader 

 commissure below the proboscis 

 tube. The " cerebral organ " 

 frequently becomes very closely 

 associated with the hinder part 

 of the dorsal ganglion (Fig. 



FIG. XIX. 



Brain of Eupolia giardii, Hubr. (from Perrier, 

 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. 



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. k). 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 Nemcrtines, in the evolution of Vertebrata, see Hubrecht 

 (18). 



