THE NEMERTIN1 



169 



below the cutis. These nerves are connected by a nerve tunic, or 

 plexus in this position, extending all round the animal. The 

 brain likewise occupies the same relative position. 



The characters upon which Biirger chiefly founds his classifica- 

 tion are (a) the somatic musculature, and (b) the relative position 

 of the lateral cords. The dermal musculature consists either of 

 two layers, as in Carinella, or of three layers, in which case a 

 second coat of longitudinal muscles is developed in the cutis, 

 external to the circular coat (Fig. X.). In these "trimyaric" 

 Nemertines the lateral nerves lie between this secondary coat of 

 longitudinal muscles and the circular muscles that is, in the same 



FIG. VIII. 



Transverse section of Carinella, (somewhat schematised, after Burger), a, epidermis ; 6, 

 basement tissue (cutis) ; c, circular muscles ; c', inner (splanchnic) coat of circular muscles ; 

 d, longitudinal muscles ; e, lateral nerve stem ; /, portion of nerve network ; i, enteron ; j (super- 

 ficial) dorsal nerve ; k, proboscidial sheath nerve ; o, portion of excretory canal ; p, lateral blood- 

 vessel ; r 1 , longitudinal muscular coat of the proboscis ; r", circular coat ; r'", epithelium of 

 proboscis ; between these two last coats lies a, nerve, above and below ; s, the proboscis sheath, 

 or muscular wall of the rhynchocoel (w) ; v, parenchyma.* 



position, with regard to the latter, as in Carinella. But in. the 

 " dimyaric " forms the nervous system may either (a) lie outside 

 the circular coat, or (b) in the substance of the somatic muscula- 

 ture, or (c) below it, in the parenchyma. 



There can be no doubt but that the first of these three condi- 

 tions is the most primitive, and the last the most recent ; and the 

 trimyaric arrangement has been derived from the dimyaric in 

 another direction. 



Using, then, the terms Anopla and Enopla as descriptive rather 

 than as taxonomic terms, the Nemertines may be divided into two 

 branches the Dimyaria and the Trimyaria ; and the former may 

 be anoplous or enoplous, as follows : 



