SEGMENTS OF TRIGEMINAL NERVE-GROUP 271 
pre-oral entosclerite, a slight movement of the eyes, laterally or 
anteriorly, owing to the flexibility of the carapace, might result as 
the consequence of their contraction. But this cannot be the main 
object of these muscles. The pre-oral entosclerite is firmly fixed to 
the camerostome, as is seen in Fig. 94, pr. ent., so that the main 
object of these muscles is, as Huxley has pointed out, the movement 
of this organ. 
In order to avoid repetition of the long name given to this muscle 
group (62) by Miss Beck, because of their position, and for other 
reasons which will appear in the sequel, I will call this group of 
muscles the group of recti muscles. These recti muscles belong 
clearly to the segments posterior to the first prosomatic or cheliceral 
segment, and represent certainly three, probably four, of these 
segments, te. belong to the segments corresponding to the second, 
third, fourth, and fifth prosomatic locomotor appendages—the endo- 
guaths of the old Eurypterids. 
The next pair of muscles is the pair of anterior dorso-plastron 
muscles (63). This muscle-pair evidently belongs to a segment pos- 
terior to the segments represented by the group already discussed, 
and belongs, therefore, in all probability to the same segment as the 
sixth pair of prosomatic appendages—the ectognaths of the old 
Eurypterids. This can be settled by considering either the nerve- 
supply or the embryological development. In the Eurypteride it 
seems most highly probable that the dorso-ventral muscles of each 
half of the segments belonging to the endognaths should be compressed 
together and separate from the dorso-ventral muscle belonging to the 
ectognathal segment, on account of the evident concentration and small 
size of the endognathal segments in contradistinction to the separate- 
ness and large size of the ectognathal segment. 
The striking peculiarity of this muscle-pair, which distinguishes it 
from all other muscles in the scorpion, is the common attachment of 
the muscles of the two sides in the mid-dorsal line, so that the pair 
of muscles forms an arch through which the alimentary canal and 
dorsal blood-vessel pass. 
The same dorso-ventral muscles are present in Phrynus, and in 
this animal the fibres of this pair of muscles (63) actually interlace 
before the attachment to the prosomatic carapace, so that the attach- 
ment of the muscle on each side overpasses the mid-dorsal line, and 
a true crossing occurs. In Fig. 108 the position of this pair of 
