336 



CH.ETOPODA. 



members of the Chsetopoda, the Arthropoda and the MoUusca. An 

 account of imperfect observations on other forms, where such have 

 been made, will be found in the systematic part of this work. 



Chsetopoda. We are indebted to Kieinenberg (No. 329) for the 

 most detailed account which we have of the development of the 

 central nervous system in the Chaetopoda. 



The supraoesophageal ganglion with the oesophageal commissure 

 developes independently of the ventral cord. It arises as an unpaired 

 thickening of the epiblast, close to the dorsal side of the oesophagus at 

 the front end of the liead (fig. 239), which becomes separated from 



the epiblast, and extends obliquely back- 

 wards and downwards in a somewhat 

 arched form ; its lower extremities being 

 somewhat swollen. The inner portion 

 of this curved rudiment becomes con- 

 verted into commissural nerve-fibres, 

 while the cells of the outer and upper 

 portion assume the characters of gan- 

 glion-cells. The commissural fibres are 

 continued doivnwards to meet the ven- 

 tral cord, but their junction with the 

 latter structure is not effected till late 

 in embryonic life. 



The ventral cord is formed by the 

 coalescence of a pair of linear cords, 

 the development of which takes place from before backwards, so that 

 when their anterior part is well developed their posterior part is 

 hardly differentiated. These cords arise, one on each side of a 

 ventral ciliated furrow, first as a single row of epiblast cells, and 

 subsequently as several rows (fig. 240, Vg). While still united to the 



external epiblast, they extend them- 

 selves below the cells lining the ventral 

 furrow, and unite into a single nervous 

 band, which however exhibits its 

 double origin by its bilobed section. 

 Before the two cords unite, the groove 

 between them becomes somewhat deep, 

 but subsequently shallows out and 

 disappears. The nervous band, before 

 separating from the epiblast, exhibits, 

 in correspondence with the mesoblastic 

 segments, alternate swellings and con- 

 strictions. The former become the 

 ganglia, and the latter the connecting 

 trunks. 



As soon as the cord becomes free from the epiblast, it becomes 

 surrounded by a sheath, formed of somatic mesoblast. In each of the 

 ganglionic enlargements there next appears on the dorsal surface a 



Fig. 239. Section thkough 

 the head of a young embryo of 

 LuMBEicus TKAPEzoiDES. (After 

 Kieinenberg.) 



e.g. cephalic ganglion ; cc. ce- 

 phalic portion of the body cavity ; 

 X. oesophagus. 



Fig. 240. Section throiigh part 

 OF the ventral wall of the trunk 



OF AN embryo of LuMBRICUS TRAPE- 



zoiDES. (After Kieinenberg.) 



m. longitudinal muscles; so. so- 

 matic mesoblast ; sp. splanchnic me- 

 soblast; hy. hypoblast; Vg. ventral 

 nerve-cord; vv. ventral vessel. 



