THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 83 



Summary. 



From the close similarity of structure and position between the branchial 

 skeleton of Limulus and of Arumocoetes, as given in the preceding chapter, it 

 logically follows that the branchiae of Ammocoetes must be homologous with the 

 branchiae of Limulus. But the respiratory apparatus of Limulus consists of 

 branchial appendages. It follows, therefore, that the branchiae of Ammocoetes. 

 and consequently of the vertebrates, must have been derived from branchial 

 appendages, and as they are internal, not external, such branchial appendages 

 must have been of the nature of ; sunk-in ' branchial appendages. Such 

 internal appendages are characteristic of the scorpion tribe, and of. perhaps, 

 the majority of the Palaeostraca, for no external respiratory appendages have 

 been discovered in any of the sea-scorpions. 



In the vertebrates — and it is especially well shown in Ammocoetes — a double 

 segmentation exists in the head-region, a body or somatic segmentation, and 

 a branchial or splanchnic segmentation, respectively expressed by the terms 

 mesomeric and branchiomeric segmentations. The nerves which supply the 

 latter segments form a very well-marked group (Charles Bell's system of lateral 

 or respiratory nerves) which do not conform to the system of spinal nerves, for 

 they do not arise from separate motor and sensory roots, but are mixed nerves 

 from the very beginning. 



The system of cranial segmental nerves is older than the spinal system, and 

 cannot, therefore, be derived from it, but can be arranged as a system supplying 

 two segments, somatic and splanchnic, which differ in the following way : Each 

 somatic segment is supplied by two roots, motor and sensory respectively, as in 

 the spinal cord segments, while each splanchnic segment possesses only one root, 

 which is mixed in function. 



The peculiarities of the grouping of the cranial segmental nerves, which 

 have hitherto been unexplained, immediately receive a straightforward and 

 satisfactory explanation if the splanchnic or branchiomeric segments owe their 

 origin to a system of appendages after the style of those of Limulus. 



In Limulus and all the Arthropoda, the segmentation is double, being com- 

 posed of (1) somatic or body-segments, constituting the mesomeric segmentation ; 

 (2) appendage-segments, which, seeing that they carry the branchiae, constitute 

 a branchiomeric segmentation. Similarly to the cranial region of the vertebrate, 

 the nerves which supply the somatic segments arise from separate sensory and 

 motor roots, while the single nerve which supplies each appendage contains all 

 the fibres for the appendage, both motor and sensory. 



It follows from this that the branchial segments supplied by the vagus 

 and glossopharyngeal nerves ought to have arisen from appendages bearing 

 branchiae. 



Although the evidence of such appendages has entirely disappeared in the 

 higher vertebrates, together with the disappearance of branchiae, and is not 

 strikingly apparent in the higher gill-bearing fishes, yet in Ammocoetes, so 

 great is the difference here from all other fishes, it is natural to describe the 

 pharyngeal or respiratory chamber as a chamber into which a symmetrical series 

 of respiratory appendages, the so-called diaphragms, are dependent. Each of 

 these appendages possesses its own mixed nerve, glossopharyngeal or vagus. 



