434 DEVELOPMENT OF ORGANS. 



a slight median furrow indicates its constitution out of two lateral cords. 

 The supra-cesophageal 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 

 determined. 



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, PalaBmon) of the two halves of each segment or 

 ganglion a space filled with finely punctated material, which is the 

 commencement of the commissural portion of the cords. The com- 

 missural 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 correspond 

 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 coalescence of two 

 structures, both however epiblastic, viz. (1) part of the superficial 

 epiblast of the procephalic lobes; (2) part of the supra-oesophageal 

 ganglia. The former gives rise to the corneal lenses, the crystal- 

 line 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 interposed. 



Of the development of the auditory and olfactory organs almost 

 nothing is known. 



Dorsal organ. In a considerable number of the Malacostraca 

 and Branchiopoda a peculiar organ is developed from the epiblast 

 in the anterior dorsal region. This organ has been called the dorsal 

 organ. It appears to be of a glandular nature, and is usually very 

 large in the embryo or larva and disappears in the adult; but in 

 some Branchiopoda it persists through life. In most cases it is un- 

 paired, but in some instances a paired organ appears to take its place. 



Various views as to its nature have been put forward. There 

 is but little doubt of its being glandular, and it is possible that it is a 

 provisional renal organ, though so far as I know concretions have not 

 yet been found in it. 



