266 THE ORIGIN OF VERTEBRATES 



oblique — an order precisely the same as that of Hensen and Volckers, 

 and of Starr. 



I conclude, from the agreement between the anatomical, physio- 

 logical, and morphological evidence, that the Illrd 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 IV., 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 Piatt, and does not 

 form any permanent muscles ; that the next four belong to the pre- 

 mandibular cavity, and the 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 

 Piatt's mandibular muscle and the superior oblique. It is, to say the 

 least of it, a striking coincidence that such an arrangement of the 

 ccelomic 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, 1 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 Vlth 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 Vlth nerve. Quite 

 recently Edgeworth lias shown that in birds and reptiles this muscle 



