38-i ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 



with the digestive portion of the intestine. The walls of the 

 two anterior thirds of the branchial cavity are strengthened 

 on each side by a series of transparent cartilaginous, highly 

 elastic, hair-like ribs, which are imbedded in their substance. 

 The upper extremities of these ribs are fixed in two streaks of 

 a tough white substance, which runs along on each side of the 

 inferior surface of the chorda dorsalis, on the sides of the 

 inferior longitudinal ligament. The inferior extremities of 

 the ribs terminate hi a more complicated manner. Each 

 alternate pair of ribs bifurcates. The inferior branch of the 

 rib on each side meets its fellow of the opposite side at an 

 angle in the median line. The superior branch curves up 

 also, and meets that of the other side. The non-bifurcated 

 ribs are shorter, and terminate in a line with the bifurcation 

 of the neighbouring pairs. There results from this arrange- 

 ment a sort of skeleton canal, the walls of which are completed 

 by membrane. There are from seventy to eighty ribs on each 

 side. Their general direction is from above downwards and 

 from before backwards, but more perpendicular than the 

 ventral bundles of the lateral muscles, with which they form 

 acute angles. Along the edges of these ribs vessels pass for 

 the performance of the respiratory function, and the canal 

 above described contains the branchial artery or heart. 



Vascular System. 



In the canal which has been described as passing along 

 the inferior wall of the branchial compartment of the intestinal 

 tube, a vessel runs. This vessel diminishes anteriorly ; and, 

 posteriorly, it also diminishes, and is lost in the direction of 

 the digestive tube. Valves, if they exist, have not been 

 detected in this tube. At the extremities of each pair of 

 bifurcated ribs, the abdominal vessel just described gives off 

 a primary branch, which passes along the edge of the rib, 

 sending secondary branches at regular intervals and at right 

 angles to the other primary branches on each side. Along 



