284 THE ORIGIN OF VERTEBRATES 



vertebrate embryology (as has been so often asserted to be the case), 

 seeing that undoubtedly the Arthropoda are an advanced stage of 

 Annelida ; and, indeed, the way is not a long one when we consider 

 Beecher's evidence that the Trilobita belong to the Phyllopoda, 

 certainly a primitive crustacean group, which Bernard derives directly 

 from the annelid group Chretopoda. If, then, these plakodal ganglia 

 indicate the former presence of appendages, we obtain this result : — 

 The foremost ganglion on each side possesses one plakodal ganglion, 

 and therefore indicates an anterior pair of appendages, possibly the 

 chelicerse. Then comes the peculiar nerve with four plakodal 

 ganglia indicating on each side four appendages close together, 

 possibly the endognaths. Then, finally, on each side, the second 

 large ganglion with two plakodal ganglia, indicating two pairs of 

 appendages, possibly the ectognaths and the metastoma. 



Summary. 



The consideration of the history of the cranial segmentation shows that 

 whereas, from the commencement of that history, the evidence for two ventral 

 segments supplied by the trigeminal nerve is clear and unmistakable, later 

 observers have tended more and more to increase the number of these segments, 

 until at the present time the evidence is in favour of at least six, probably seven, 

 as the number of segments supplied by the motor part of the trigeminal. 



So. also, the original evidence for the number of dorsal or somatic segments 

 limits the number to three, innervated respectively by the oculomotor (III.), 

 trochlear (IV.), and abducens (VI.) nerves, or rather two, since the last nerve 

 belongs to the facial segment. The muscles which these three nerves supply 

 are derived respectively from the walls of the premandibular. mandibular, and 

 hyoid coelomic cavities. 



Later evidence points strongly to the conclusion that the oculomotor nerve 

 and the premandibular cavity represent not one segment but the fusion of 

 four, while the mandibular cavity represents two segments. In addition to 

 these. Miss Piatt has discovered a still more anterior head-cavity, which she has 

 named the anterior cavity, so that the pro-otic segments on this reckoning are 

 seven in number, viz. : (1) the anterior cavity, (2, 3, 4, 5) the premandibular cavity, 

 (6, 7) the mandibular cavity. The somatic muscles belonging to these dorsal 

 segments are the eye-muscles, which are all dorso-ventral in position, and are 

 not the same as the longitudinal somatic muscles, but belong to a distinct dorso- 

 ventral segmental group, the only representative of which at present known in 

 the mesosomatic region is the external rectus innervated by the Vlth nerve. 



These head-cavities, and these muscles of the vertebrate, resemble the 

 corresponding cavities and muscles of the invertebrate to an extraordinary 



