NERVOUS SYSTEM. 407 



Discophora, the Gephyrea, the Tracheata, and the Crustacea, 

 the various small Arthropodan phyla (Pcecilopoda, Pycnognida, 

 Tardigrada, &c.), the Chaetognatha (?), and the Myzostomea, 

 probably belongs to this type. 



The nervous system of the Echinodermata cannot be reduced 

 to this form ; nor in the present state of our knowledge can that 

 of the Nematelminthes or Enteropneusta. 



It is only in the case of members of the former set of groups 

 that any adequate observations have yet been made on the 

 development of the nervous system, and even in the case of 

 these groups observations which have any claim to completeness 

 are confined to certain members of the Chaetopoda, the Arthro- 

 poda and the Mollusca. An account of imperfect observations 

 on other forms, where such have been made, will be found in the 

 systematic part of this work. 



Chaetopoda. We are indebted to Kleinenberg (No. 329) for 

 the most detailed account which we have 

 of the development of the central nervous 

 system in the Chaetopoda. 



The supracesophageal ganglion with 

 the cesophageal commissure developes in- 

 dependently of the ventral cord. It arises 

 as an unpaired thickening of the epiblast, p IG- 239 . SECTION 

 close to the dorsal side of the oesophagus THROUGH THE HEAD OF 



A 'YOUNG EMBRYO OF 



at the front end of the head (fig. 239), LUMBRICUS TRAPEZOIDES. 

 which becomes separated from the epi- < After Kleinenber s-) 



, e.g. cephalic ganglion ; 



blast, and extends obliquely backwards CCi cephalic portion of the 

 and downwards in a somewhat arched body cavity ;*. oesophagus. 

 form ; its lower extremities being somewhat swollen. The 

 inner portion of this curved rudiment becomes converted into 

 commissural nerve-fibres, while the cells of the outer and upper 

 portion assume the characters of ganglion-cells. The com- 

 missural fibres are continued downwards to meet the ventral 

 chord, 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 



