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 

 tliem 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, i.e. the second, third, fourth, 

 fifth pairs of prosomatic appendages ; the premandibular cavity, there- 

 fore, represents the second ccelomic cavity in Limulus, which, accord- 

 ing to Kishinouye, is the sole representative^ 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, i.e. 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 Piatt, 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 coelomic cavity. They represent, according to my 

 scheme, the segments belonging to the sixth pair of prosomatic 

 appendages or ectognaths, and the seventh pair, i.e. the chilaria or 

 metastoma. 



The first part, then, of the mandibular cavity represents the third 

 coelomic 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 ((33). 



The second part of the mandibular cavity represents the 4th 



T 



