i8o THE NEMERTINI 



point for this proboscis (see p. 17). Nevertheless, it is a very 

 long step from the one to the other, and no intermediate stages are 

 known ; for in the Proboscidae there is nothing comparable to a 

 rhynchocoel, which must have been developed pari passu with the 

 elongation of the eversible organ as the only means of everting 

 the proboscis probably in the same way in which it is developed 

 ontogenetically. Burger has suggested that it represents the 

 pharynx of the Turbellaria, and sees in those genera in which the 

 rhynchostome is coincident with the mouth the original condition. 

 But apart from the difficulty of comparing structurally the pro- 

 boscis with the tubular pharynx, it is extremely improbable that 

 the Metanemertini in which alone this condition is realised 

 should have retained a primitive condition in this respect, whilst 

 in the remainder of their anatomy they are so evidently much 

 less primitive than the Proto- or Meso-nemertini, in which there is 

 no sort of connection between the two organs. 



The nervous system of the Nemertines (see 6, 16, 17) is primi- 

 tively in the form of a network, as in Turbellaria; but in the 

 lower orders this network retains a more superficial, and phylo- 

 genetically, more archaic position than in that Class ; for in the 

 Protonemertini it lies among the bases of the epidermal cells 

 (Carinina), as in the Coelentera (Fig. XVIII. 1, e) ; in Carinella and 

 Hubrechtia it sinks through the basement membrane so as to lie 

 immediately below it ; in the Mesonemertini this process continues, 

 so that in Carinoma the nerves lie outside the circular muscles, in 

 the region of the foregut, but come to occupy a deeper position, 

 viz. in the longitudinal coat, in the posterior region of the body 

 a position which they occupy throughout the body in Cephalothrix 

 (Fig. XVIII. 2). Finally, in the Metanemertini the nervous 

 system has sunk into the parenchyma, and occupies the same 

 position as in the Turbellaria (Fig. XVIII. 3). The case of the 

 Heteronemertini must be considered apart, for although the nerve 

 cords lie between the muscular coats, this position is not so much 

 the result of sinking inwards, as of being thrust downwards by 

 the development of a new muscular layer outside the circular 

 coat (Fig. X.) ; indeed, with regard to the latter, the nerves 

 occupy the same position as in Carinella. 



The nervous network, even in the lowest Nemertines, presents 

 ^i differentiation into a central and peripheral system, though it 

 is impossible to draw a hard-and-fast line between them ; for 

 certain tracts have, as in the Turbellaria, become larger and more 

 definite than others, giving rise to longitudinal nerve stems ; but 

 of these only three are recognisable, namely, a pair of stout lateral 

 stems, and a smaller median dorsal nerve. These traverse the 

 whole length of the body, and are connected at each end by a trans- 

 verse, supra-enteric commissure or anastomosis. The elongated 



