SEGMENTS OF TRIGEMINAL NERVE-GROUP 273 
oculomotor nerve from the dorsal mesoderm segments, ‘These four 
segments have been so fused together that van Wijhe looked upon 
them as a single segment, and the premandibular cavity as the cavity 
of a single segment. They represent, according to my scheme, the 
segments belonging to the endognaths, 7.c. the second, third, fourth, 
fifth pairs of prosomatic appendages ; the premandibular cavity, there- 
fore, represents the second coelomic cavity in Limulus, which, accord- 
ing to Kishinouye, is the sole representative of the coelomic cavities 
of the second, third, fourth, fifth prosomatic segments. The muscles 
derived from the ventral mesoderm-segments represent the muscles 
of these appendages, which therefore dwindle and disappear in the 
vertebrate, with the possible exception of the muscles innervated by 
the descending root of the trigeminal. The muscles derived from 
the dorsal mesoderm-segments, %.¢. the eye-muscles supplied by the 
oculomotor nerve, represent the dorso-ventral somatic muscles of these 
four segments, muscles which are represented in the scorpion by the 
recti group of muscles, i.e. the median dorso-preoral-entosclerite 
muscles (62). 
Then came two segments, the mandibular, in which muscles are 
formed both from the ventral and from the dorsal mesoderm-segments. 
From the former arose the main mass of muscles innervated by 
the motor root of the trigeminal, from the latter the superior oblique 
muscle and the mandibular muscle of Miss Platt, of which the former 
alone survives in the adult condition. These two segments are looked 
upon as a single segment by van Wijhe, of which the mandibular 
cavity is the ccelomic cavity. They represent, according to my 
scheme, the segments belonging to the sixth pair of prosomatic 
appendages or ectognaths, and the seventh pair, ¢.e. the chilaria or 
metastoma. 
The first part, then, of the mandibular cavity represents the third 
ceelomic cavity in Limulus and the muscles derived from the ventral 
mesoderm, in all probability the muscles of the tongue in the 
lamprey (cf. Chap. IX.), which represents the ectognaths or sixth 
pair of prosomatic appendages, while the muscles derived from the 
dorsal mesoderm, i.e, the superior oblique muscles, represent the 
dorso-ventral somatic muscles of this segment, muscles which are 
represented in the scorpion group by the pair of anterior dorso- 
plastron or oblique muscles (63). 
“The second part of the mandibular cavity represents the 4th 
T 
