ANNELIDA 287 



should be noted at the beginning that upon these matters 

 there prevails as yet among authors but little clearness, and 

 agreement to only a limited extent. 



Ectodermal Structures. 



The epidermis of the larva and of the adult worm arises 

 directly from the embryonal ectoderm, its cells multiplying 

 greatly, and becoming much flattened. 



The setigerous sacs arise, according to the concurrent state- 

 ments of KowALEVSKY, Vejdovsky, and Bergh, as club-shaped 

 ingrowths of the epidermis, inside of which the setse are 

 secreted. According to other descriptions, the setigerous sacs 

 originate from the mesoderm. 



Nervous System and Sensory Organs. — In considering the 

 origin of the nervous system it seems necessary to separate 

 the supra-oesophageal ganglion from the ventral cord. Both 

 arise as thickenings of the ectoderm (Fig. 133 C), the ven- 

 tral chain of ganglia either as a longitudinal, unpaired 

 or as a paired thickening, which detaches itself from the 

 ectoderm, and moves to the inside, where it may be sur- 

 rounded by mesoderm (Kowalevsky). The further develop- 

 ment proceeds from in front backwards. Opinions are very 

 far apart regarding the origin of the supra-oesophageal 

 ganglion, and especially its connection with the ventral 

 chain of ganglia. 



In Hatschek's opinion, there first arises an ectodermal thickening at 

 the head end of the embryo : the apical plate. From this the ectodermal 

 thickening progresses backwards in the form of two cords, which extend 

 on either side of the mouth. From the oesophageal connectives thus 

 formed, the thickening process continues further and further. In this way 

 the two lateral cords of the ventral nerve-trunk are formed, and in addition 

 a groove-like invagination, lying in the longitudinal median line (similar 

 to the medullary tube of vertebrates), takes part in the formation of the 

 ventral chain of ganglia. Hatschek defends the view that the entire 

 nervous system arises from a single fundament. In this he relies mainly 

 upon his embryological investigations on Criodrilus and Polygordius, and 

 furthermore on the comparative anatomical conditions in Protodrilus, in 

 which Archiannelid the oesophageal connectives are said to remain 

 throughout life in connection with the body epithelium as ectodermal 

 thickenings. 



