THE EVIDENCE OF THE RESPIRATORY APPARATUS 183 
SUMMARY. 
From the close similarity of structure and position between-the branchial 
skeleton of Limulus and of Ammoccetes, as given in the preceding chapter, it 
logically follows that the branchiz of Ammocctes must be homologous with the 
branchie of Limulus. But the respiratory apparatus of Limulus consists of 
branchial appendages. It follows, therefore, that the branchiee of Ammoceetes, 
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 Paleostraca, for no external respiratory appendages have 
been discovered in any of the sea-scorpions. , 
In the vertebrates —and it is especially well shown in Ammocotes—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 branchie, 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 
branchiee. 
Although the evidence of such appendages has entirely disappeared in the 
higher vertebrates, together with the disappearance of branchiew, and is not 
strikingly apparent in the higher gill-bearing fishes, yet in Ammoccetes, 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. Hach of 
these appendages possesses its own mixed nerve, glossopharyngeal or vagus, 
