522 DEVELOPMENT OF ORGANS. 



median furrow indicates its constitution out of two lateral cords. The 

 supra-oesophageal ganglia are stated to be developed quite simply as a pair 

 of thickenings of the procephalic lobes, but whether they are from the 

 first continuous with the ventral cord does not appear to have been deter- 

 mined. 



The later stages in the differentiation of the ventral cord are, 

 so far as is known, very similar throughout the Crustacea. The 

 ventral cord is, as has been stated, at first unsegmented (fig. 241 

 A, vg\ but soon becomes divided by a series of constrictions into 

 as many ganglia as there are pairs of appendages or segments 

 (fig. 241 B, vg). 



There appears either on the ventral side (Oniscus) or in the 

 centre (Astacus, Palsemon) of the two halves of each segment or 

 ganglion a space filled with finely punctuated material, which is 

 the commencement of the commissural portion of the cords. 

 The commissural tissue soon becomes continuous through the 

 length of the ventral cord, and is also prolonged into the supra- 

 cesophageal ganglia. 



After the formation of the commissural tissue the remaining 

 cells of the cord form the true ganglion cells. A gradual 

 separation of the ganglia next takes place, and the cells become 

 confined to the ganglia, which are finally only connected by a 

 double band of commissural tissue. The commissural tissue not 

 only gives rise to the longitudinal cords connecting the successive 

 ganglia, but also to the transverse commissures which unite the 

 two halves of the individual ganglia. 



The ganglia usually, if not always, appear at first to corre- 

 spond in number with the segments, and the smaller number so 

 often present in the adult is due to the coalescence of originally 

 distinct ganglia. 



Organs of special sense. Comparatively little is known on 

 this head. The compound eyes are developed from the coales- 

 cence of two structures, both however epiblastic, viz. (i) part of 

 the superficial epiblast of the procephalic lobes ; (2) part of the 

 supra-cesophageal ganglia. The former gives rise to the corneal 

 lenses, the crystalline cones, and the pigment surrounding 

 them ; the latter to the rhabdoms and the cells which encircle 

 them. Between these two parts a mesoblastic pigment is inter- 

 posed. 



