518 PROFESSOR E, RAY LANKESTER, 
border of the thirteenth segment (seventh of the abdominal 
series) to the soft membrane which forms the hinge of the 
postanal spine. 
In the embryo Limulus, however, this area is further seg- 
mented. We do not find the five segments of the Scorpion, 
but we find two of which (as segments) no indication is left 
in the adult, and the foremost of these carries a movable 
spine on each side like those in front of it. 
The anterior margin of the segment or tract of the body 
which carries the anus appears to be uniformly in Arthro- 
poda, and in some other segmented animals, the part from 
which new segments grow and become individualised, and 
it is to this tract of the body including its pree- and post- 
anal regions that the name “ telson” is applicable as, for 
example, in the Lobster. It not unfrequently happens that 
this segment-producing region does not produce the full 
number of segments in given examples of an Arthropodous 
class, which is characteristic of the majority or of the more 
fully segmented members of the class. Thus, both in 
Crustacea and Arachnida we find numerous forms with a 
reduced number of abdominal segments. Usually, however, 
as in the spiders, the embryo exhibits at some time of its 
development the full complement of segments, the hinder- 
most of which subsequently become obliterated by fusion or 
atrophy. Limulus so far conforms to this plan as to show | 
the segmental potentiality of its preanal area, but fails to 
exhibit to the observer the full complement of segments even 
as a temporary arrangement of its living substance. 
Accordingly the whole area posterior to the ridge mark- 
ing the posterior border of the thirteenth segment may 
be regarded in Limulus as belonging to the “telson,” or 
area of potential segmentation, a certain reservation being 
observed in respect to the one or two minute segments 
which appeared and disappeared in this region in the 
embryo. 
We may, when comparing this condition of things with 
that exhibited by the Scorpion, either consider the telsonic 
area and spine of Limulus as representing the five cylin- 
drical segments and the sting of the Scorpion in an unseg- 
mented state, or we may insist rather upon the actuality 
than the protentiality, and identify the telson or fifth of the 
cylindrical segments of the Scorpion (viz. that carrying the 
anus), and the postanal spine with the telsonic area and 
spine of Limulus, whilst regarding the four anterior cylin- 
1 Note also the evanescent character of the three last segments of Thely- 
phonus (fig. 12). 
