No. 3-] STUDIES ON THE HETERONEMERTINI. 427 



ture as in Cerebratuhis ; but in L. sp., in addition to this second 

 commissure, there is also a third ventral commissure (absent in 

 the two preceding species) which is placed two sections (about 

 "/fi) behind the second. 



The paired oesophageal nerves of Cerebratuhis lacteus ("vagus 

 nerves," according to Hubrecht, who sought to homologize 

 them with the vertebrate vagi), I find to have three commis- 

 sures, as Burger ('90b) has described for C. margmatus; while 

 Coe ('95a), who also studied C. lacteus, saw only the third 

 (largest) commissure. Each oesophageal nerve has its origin 

 on the median side of the fibrous core of the ventral brain lobe, 

 a short distance behind the first ventral commissure of the 

 latter; the nerve is therefore, in point of origin, a posterior 

 continuation of part of the fibrous core of the ventral lobe. In 

 this core the nerve has its own sheath, which is a derivative 

 of the inner neurilemma of the former. About five sections 

 behind the second ventral commissure of the brain lies Wiq first 

 commissure of the oesophageal nerves, the latter commissure 

 being but little thicker than the former. From this point on, 

 these nerves are situated outside of the fibrous core of the brain 

 lobe. Three sections behind the first lies the second commis- 

 sure of the oesophageal nerves, which has the same diameter as 

 the first. An equal distance behind the second is situated the 

 third cojn7izissure, which is the last and much the largest of the 

 three, both in length and diameter. Behind this last commis- 

 sure the oesophageal nerves diverge from each other, as de- 

 scribed by Hubrecht and Burger, each passing downwards and 

 outwards to occupy a latero-ventral position in the inner longi- 

 tudinal muscle layer of the body wall, close to the oesophagus; 

 in their course they give off a number of smaller nerves. The 

 oesophageal nerves, both in the fibrous core of the brain as well 

 as posteriorly, are enveloped by a neurilemmatic sheath, beneath 

 which a few inner neuroglia cells lie (Fig. 43, Oes. N.). Gan- 

 glion cells are present only around the commissures of the 

 nerves, most abundantly (though only in a single layer) around 

 the third; these all are modifications of cells III of the brain 

 lobes and have been already described (Fig. 20). The neuri- 

 lemmatic sheath is absent only around the commissures. 



