164 CRUSTACEA. 



show, according to Keiciienbach, in their development, great agree- 

 ment both inter se and with the ganglia of the ventral cord. In 

 each of these pairs of ganglia we can distinguish lateral strands and 

 a median strand ; the lateral strands, in cross section, are seen to be 

 broken up into three parts, as are the ganglia of the ventral cord. 

 The median strand, however, varies in different regions. In the 

 region of the optic ganglion, the two halves of the median strand 

 shift far apart, and enter separately into connection with the corre- 

 sponding ganglia. In the region of the procerebrum and the 

 antennular ganglion, on the other hand, is found that median 

 invagination of this strand which doubtless leads to the develop- 

 ment of the commissural portions of the brain. In the region of 

 the antennal ganglion, again, there is no median invagination. 

 Eeichenbach believes that this invagination has shifted forwards 

 and is represented by that established between the antennular 

 ganglia. If, however, we l^elieve that the transverse commissure 

 between the antennal ganglia was originally post-oral, and is perhaps 

 still to be looked for in such a position (Claus, No. 78), we 'shall 

 not be surprised at the absence of the median invagination between 

 these ganglia. In later stages, the invaginations of the median 

 strand are no longer distinct from one another in the region of the 

 procerebrum and the antennular ganglion, a closer union between 

 the different parts of the brain being then generally apparent. The 

 procerebrum, according to Eeichenbach, gives origin principally to 

 the " anterior cerebral swelling," while the antennular ganglion is 

 connected with the development of the "lateral swellings" (Krieger,. 

 Dietl). 



These observations of Eeichenbach are to a certain extent in 

 agreement with those of Ivingsley, who found in Craiigon, apart 

 from the optic ganglia, three consecutive pairs of ganglia taking 

 part in the formation of the brain. The most anterior of these 

 (the procerebrum) Kingsley, however, regarded from the first as an 

 independent structure, it alone being originally preoral and homo- 

 logous with the supraoesophageal ganglion of the Annelida. The 

 two pairs of ganglia which folloAV (the antennular and antennal 

 ganglia) are, when first developed, post-oral in position, and must- 

 thus be considered as "anc-lia of the ventral cord drawn into the 



O o 



cephalic region. 



The above oliservations lead us uj) to the question of the primary segmentation 

 of the pre-ora] portion of tlie head iu the Crustacea. Reichenbach, witli 

 whom also Nusbaum (No. 39) is in essential agreement, has been led by his- 



