NEMERTINI 229 



The origin of the different organs in the Nemerteans with direct deve- 

 lopment has recently been studied by Salensky (No. 24) on Monopora. It 

 corresponds essentially with what we have learned as occurring in the 

 indirect process of development. At the anterior end the central nervous 

 system arises in the form of two ectodermal thickenings, which soon 

 become detached from their connection with the ectoderm. The funda- 

 ments of the two brain ganglia grow backwards as the two lateral nerves. 



The proboscis and the oesophagus arise in the vicinity of the ganglionic 

 fundaments, both of them as bud-like thickenings of the ectoderm and 

 both of very similar appearance. The proboscis in this form lies, even 

 in the adult animal, very close to the oesophagus, whereby the relation of 

 the fundaments of the two organs is explained. The proboscis, which 

 lies above the oesophagus, opens with it into a common atrium. In spite 

 of this, however, relations [genetic] between oesophagus and proboscis 

 should hardly be sought for here, as Hoffmann and Balfoue supposed ; 

 but the condition in Monopora is much more likely of a secondary nature. 

 The proboscis, at first located at the anterior end of the body, was only 

 subsequently [phylogenetically] united with the oesophagus by moving 

 backwards. Moreover, the union is a very loose one, for the proboscis 

 and oesophagus do not actually unite, but rather open independently of 

 each other into the common atrium. 



In the further course of development the fundament of the oesophagus 

 becomes hollow and unites with the intestine. The latter, after the 

 closure of the blastopore, consists of a closed sac. Entodermal cells 

 migrate into its lumen, thereby entirely filling it. Later they become 

 arranged into an epithelium, and then the oesophagus also unites with 

 the wall of the intestine. Afterwards the anus is formed. 



The proboscis in this case also is composed of ectoderm and mesoderm, 

 the latter giving rise to the envelope of the ectodermal invagination and 

 to the proboscis-sheath. Salensky argues for a formation of the body- 

 cavity by means of a splitting of the middle germ-layer in Monopora also. 

 Even before the appearance of the body-cavity, two lateral bloodvessels 

 and one dorsal are said to be formed, corresponding to the conditions of 

 the adult animal. To all appearances they owe their origin to the inner 

 part of the mesoderm, for they are located in the vicinity of the intestine. 



We find no statements in Salensky regarding the formation of the 

 genital system. 



General Considerations. 



In conclusion, we must once more point out how closely 

 the different modes of development of the Nemerteans ap- 

 proach one another. In the pilidium the worm arises by 

 the formation of four vesicular ectodermal invaginations, 

 which assume a discoid shape, and, growing around the 



