500 ALLIS. [Vou. II. 
The position of these organs in the canals, and their method 
of innervation, is shown in the diagrammatic drawing, Fig. 49, 
Plate XL: 
In this diagram two sense-organs, or groups of organs, are 
shown between the first trunks of the supra-orbital line on either 
side of the head, and one in this same line between trunks 18 
and 19, directly opposite the point where the supratemporal 
cross-commissure joins it, with no trunk system between it and 
the first group of organs in this commissure. These two trunks 
and pores have disappeared, the position of the organs and the 
development of the system alone showing that they at one time 
existed, There is but one group of organs in the infra-orbital 
canal on each side of the double trunk 17-17, and only one 
between it and trunk 16 operculo-mandibular, showing that the 
double trunk, although displaced in the adult, belongs primarily 
at the point of union of the two canals. There is one group of 
organs on each side of the double trunk 15-7, between it and 
the next trunk, in the infra-orbital and supra-orbital canals, 
showing that the displaced primary branch, which in the adult 
issues between the squamosal and postfrontal, is in reality a 
part of the double trunk, and not a separate system. There is 
also but one group on each side of the compound trunk 11 of 
the operculo-mandibular line, between it and the next trunks 
of that line, and but one on each side of the median system of 
the supratemporal cross-commissure. 
The fifth group of the infra-orbital line is appacedly out of 
place, lying in what seems to be the trunk of system 6, above 
the point where the anterior commissure joins the suborbital 
part of the main canal. This is due to the fact that the end of 
the suborbital canal, beyond the point where the commissure 
joins it, has taken the direction of the commissure instead of 
retaining that of its own canal; so that what appears as the 
trunk of system 6 is in reality this trunk and the extreme ante- 
rior end of the main canal, and it is in this anterior end, and not 
in trunk 6, that group 5 lies. Moreover, system 6 being prop- 
erly a terminal system in which the trunk continues the line of 
the main canal, there is nothing to indicate where it begins and 
the canal ends. Organs 1 and 7 supra-orbital and 1 operculo- 
mandibular occupy strictly analogous positions, apparently lying 
in the trunks of the neighboring terminal systems. 
