NERVOUS SYSTEM OF SCORPION. 43 



Laurie (1896, I and 2) described the variation in positions of 

 the abdominal ganglia in different species of scorpion. 



Police described the anatomy of the nervous system of the 

 scorpion (1901, i) and the histology of the subintestinal portion 

 (1901, 2). He described the typical neuromere as a pair of 

 ganglia fused in the mid-line and giving off two pairs of nerves, 

 " external" and "internal." The longitudinal connectives con- 

 tinue through the fibrous substance of the nerve center as a pair 

 of "central columns," which are connected by two tracts of 

 commissural fibers, anterior and posterior. There is also a pair 

 of "ventral columns." The central columns send nerve fibers 

 into both pairs of nerves, those going into the internal nerves 

 leaving the columns at the posterior cross-commissure. The 

 ventral columns send nerve fibers into the external nerves. This 

 classification of the nerves as "external" and " internal" is un- 

 fortunate, for, although in some abdominal neuromeres the ex- 

 ternal nerves do arise from a broader part of the nerve center 

 than the internal, and are properly described by the name, in the 

 thoracic neuromeres they do not. They had better be described 

 as anterior and posterior, since during the early embryology they 

 all conform to this description. 



Lankester (1904) held the view that there is only one pre- 



cheliceral neuromere. 



DESCRIPTIVE. 



Stage i. (Fig. I.) This is the earliest stage in which the 

 neuromeres can be clearly distinguished from the rudiments of 

 the appendages. In addition to the telson, which is not con- 

 sidered as a segment in the strict sense of the term, eleven seg- 

 ments have been formed. Appendages are appearing on segments 

 two to seven. Neuromeres can be distinguished on segments 

 one to seven, being represented by paired thickenings of the 

 ectoderm indented with small pits. The first neuromere, com- 

 posed of a pair of "cephalic lobes," is much larger than any of 

 the others. The oral invagination (o) is appearing in the first 

 segment. From the mouth backward to the tenth segment in 

 this stage is a median depression, the longitudinal furrow of Met- 

 schnikoff or median furrow of Patten. The depression is caused 

 by the ectoderm being thinner in this region. 



