Royal Society of Edinburgh. 34? 



germs of interspinous bones and fin- rays — along the base of the dor- 

 sal and anal fins. The tissue of this neuro- skeleton was not even 

 cartilaginous, consisting merely of membrane and globular nuclei, 

 derived from the original elementary cells. The splanchno- skeleton 

 consisted of a hyoid apparatus, and of 70 to 80 pairs of elastic fila- 

 mentous ribs. The hyoid apparatus — in tw^o divisions, with 17 pieces 

 in each — exhibited 34 rays, pointing inwards, and each springing from 

 one of the 34 basal elements of the hyoid bone. These rays the 

 author looked upon as developments of the tubercles and teeth of the 

 central aspect of the branchial apparatus of the higher fishes, and not 

 as branchiostegal rays. The ribs were enveloped in the mucous 

 membrane of the intestine, and each alternate pair bifurcated below, 

 to enclose the abdominal longitudinal vessel or heart. From these 

 circumstances, and from other considerations, the author looked upon 

 the ribs of Amphioxus not as true ribs, but as splanchno-ribs — repe- 

 titions of the hyoid bone — analogues of the tracheal and bronchial 

 cartilages of the higher Vertebrata. The tissue of the splanchno- 

 skeleton is more advanced than that of the neuro- skeleton ; the ribs 

 are cartilaginous ; the hyoid bones hollow cartilages, with isolated 

 cells or nuclei in their interior. 



The nervous system presents nothing more than a spinal cord, 

 without a trace of cerebral development, and from 60 to 70 pairs of 

 spinal nerves. The spinal cord was in the form of a ribbon, pointed 

 at both ends, with a dorsal median groove, and a line of black or 

 grey matter ; was composed of nucleated cells, without tubes or 

 fibres, and gave origin to the nerves in single roots only. The nerves 

 were all symmetrical, dividing into dorsal and ventral branches. The 

 second pair sent back a dorsal and a ventral branch, to join the cor- 

 responding branches of the other nerves, along the sides of the body, 

 and along the bases of the dorsal and anal fins ; from which distri- 

 bution the author was inclined to believe, that although the second 

 pair in Amphioxus presented certain resemblances to the vagus, it 

 was, in reality, the trifacial. 



The vascular system consisted of a straight abdominal vessel, the 

 branchial artery or heart, without any trace of valves or division into 

 cavities. This vessel sent off lateral branches, which passing up on 

 the internal surface of the intestine, along the ribs, communicated by 

 a capillary respiratory system of vessels with a dorsal trunk or aorta. : 



The intestinal tube was straight from mouth to anus, its anterior 

 half dilated, strengthened by ribs as described above, and its entrance 

 guarded by the hyoid rays. This dilated portion of the canal received 

 sea-water, as in the Ascidice, to act on the respiratory vascular rami- 

 fications on its internal surface, which is undoubtedly ciliated in the 

 living animal. The digestive portion of the canal is narrow, and 

 presents not a trace of a liver, or of any other assistant chylo-poietic 

 viscus. 



As there was no trace of branchial fissures — as the ribs were too 

 numerous to be looked upon as true branchial arches (branchial 

 arches alternating with branchial fissures) — and as the other organic 

 systems were in the condition of those of an embryo before the ap- 

 pearance of branchial clefts, the author was led to the conclusion 



