SEGMENTS OF TRIGEMINAL NERVE-GROUP 271 



pry-oral eutosclerite, 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, i.e. belong to the segments corresponding to the second, 

 third, fourth, and fifth prosomatic locomotor appendages — the endo- 

 gnaths 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 Eurypterids 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 



