NEMERTINI 223 



their connection with the primary ectoderm of the pilidium, in .order to 

 fuse with that of the somatic discs (Fig. 104 B). Thus they are said to 

 be formed directly as jjarts of the pilidium. 



The nervous system of the young worm makes its appearance in the 

 form of two ectodermal thickenings (Fig. 104 N), which arise in the 

 region of the anterior pair of discs on either side of the invagination of 

 the proboscis. At this place the ectoderm cells are differentiated into 

 several layers, of which the more superficial are said to become the body 

 epithelium and the ganglionic cells, the deeper, the Punktmhstanz. 

 The anterior thickened parts of the fundaments correspond to the brain, 

 and their backward prolongations to the lateral nerve-trunks (Fig. 104 A 

 and B). According to this, the fundament of the central nervous system 

 would have nothing to do with the apical plate of the larva. 



Even before the discs had separated from the ectoderm, mesenchyma 

 cells were applied to their inner (deeper) layer ; and since such cells were 

 also found in the region of the larval intestine, a considerable number 

 of them came to be enclosed within the worm (Butschli, Salensky). 

 Like the separate fundaments of the cephalic and somatic parts, the 

 fundament of the mesoderm is double. In the first place, a mass of 

 mesenchyma cells is formed on each of the two prostomial discs, and a 

 similar one at the apex of the invagination of the proboscis. It could 

 not be determined whether the latter originated from the former. Then 

 each disc has its own mesenchyma layer, which likewise has arisen by 

 an accumulation of mesenchyma cells. The anterior and posterior parts 

 of the body are established, therefore, quite independently. The mesen- 

 chyma of the trunk is said by Salensky to split into two layers, one of 

 which is applied to the intestine as the splanchnic layer, the other to the 

 body-wall as the somatic layer. A kind of ccelom thus arises, which, to 

 be sure, subsequently becomes reduced and breaks up into small cavities, 

 owing to the cells of both layers sending out processes which unite with 

 one another. In the head that part of the mesoderm which is ajaplied to 

 the prostomial discs becomes the musculature, whereas the layer in con- 

 tact with the proboscis splits into two cell-layers, one of which is applied 

 to the proboscis, while the other forms the sheath of the proboscis. 

 Accordingly the cavity of the proboscis-sheath would be a portion of the 

 ccBlom (Salensky). The proboscis and its sheath attain their subsequent 

 great length by growing backwards (Fig. 104 B). 



[The results of a recent investigation by Bijrger (Appendix to Literature 

 on Nemertini) differ in several particulars from the account given above. 

 The formation, and especially the differentiation, of the head- and trunk- 

 discs, the formation of the head itself, and the development of the nerv- 

 ous system are there described quite differently. The musculature of 

 the dermo-muscular sac appears to be of double origin, inasmuch as the 

 outer layer of it arises from the ectoderm, but the remaining portion 

 from the mesoderm. 



Similar statements are also made regarding the Annelids. — K.] 



