266 THE ORIGIN OF VERTEBRATES 
oblique—an order precisely the same as that of Hensen and Vélckers, 
and of Starr. 
I conclude, from the agreement between the anatomical, physio- 
logical, and morphological evidence, that the I[Ird and IVth nerves 
contain the motor somatic nerves belonging to the same segments as 
the motor trigeminal, in other words, to the prosomatic segments, so 
that the eye-muscles, innervated by III. and 1V., represent segmental 
muscles belonging to the prosoma. Further, I conclude that originally 
there were seven prosomatic segments, the first of which is repre- 
sented by the anterior cavity described by Miss Platt, and does not 
form any permanent muscles; that the next four belong to the pre- 
mandibular cavity, and +he muscles formed are the superior rectus, 
internal rectus, inferior rectus, and inferior oblique; and that the last 
two belong to the mandibular cavity, the muscles formed being Miss 
Platt’s mandibular muscle and the superior oblique. It is, to say the 
least of it, a striking coincidence that such an arrangement of the 
celomic cavities as here given should be so closely mimicked by 
the arrangement in the prosomatic region of Limulus as already 
mentioned ; it suggests inevitably that the head-cavities of the verte- 
brate are nothing more than the prosomatic and mesosomatic 
segmental ccelomic cavities, as found in animals such as Limulus. 
In the table on p. 253, I have inserted the segments in the vertebrate 
for comparison with those of Limulus. 
Before we can come to any conclusion as to the original position 
of these eye-muscles, it is necessary to consider the VIth nerve and 
the external rectus muscle. This nerve and this muscle belong to 
van Wijhe’s 4th segment. The muscle is, therefore, the somatic 
segmental muscle belonging to the same segment as the facial and is, 
in fact, a segmental muscle belonging not to the prosoma, but to the 
mesosoma. Neal comes to the conclusion that the existing abducens 
is the only root which remains permanent among a whole series of 
corresponding ventral roots belonging to the opisthotic segments, and 
further points out that the external rectus was originally an opis- 
thotic muscle which has taken up a pro-otic position, or, translating 
this statement into the language of Limulus, etc., it is a. mesosomatic 
muscle which has taken up a prosomatic position. 
There is, however, another muscle—the Retractor oculi—belonging 
to the same group which is innervated by the VIth nerve. Quite 
recently Edgeworth has shown that in birds and reptiles this muscle 
