MR GOODSIR ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 255 



articulated together. From each of these pieces, except the first, a ray proceeds, 

 those at the extremities of the two divisions being shorter than those at the centre. 

 The anterior extremities of the two divisions, or branches of the apparatus, meet at 

 the anterior superior angle of the mouth ; and the two posterior, after curving for- 

 ward, meet at the posterior inferior angle. The various pieces of which this appa- 

 ratus consists have the consistence of cartilage. They are hollow along the bases, 

 and to the points of the rays. Their cavities appear to be full of a transparent 

 fluid, containing here and there masses of globular cells, exactly similar to those 

 in the interspinous bones. This part of the skeleton is covered by the integuments, 

 and by the membrane of the branchial cavity. A fringe of the integument sur- 

 rounds the hyoid rays, extending a little beyond their bases. This fringe must 

 be considered as the lip or margin of the mouth, the hyoid rays, although occa- 

 sionally dependent, belonging properly to the cavity of the mouth. The rami of 

 the hyoid are brought together, and the mouth closed, by the transverse muscle 

 formerly described. 



Immediately behind the hyoid apparatus the branchial cavity commences, 

 and continues as a dilated tube, which at last contracts, and becomes continuous, 

 as formerly described, 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 imbed- 

 ded 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 in 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 arrangement 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 per- 

 formance 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 dimi- 

 nishes 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. 



VOL. xv. PART i. 3 z 



