ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 383 



the intestinal tube, on the walls of which a peculiar vascular 

 arrangement exists for the aeration of the blood, and the com- 

 plicated skeleton superadded, for the efficient performance of 

 that function. 



In connection with the respiratory apparatus, I shall, as 

 formerly proposed, describe the splanchno-skeleton. This 

 division of the osseous system consists of a hyoid apparatus 

 and of a range of branchial ribs. 



The hyoid apparatus supports the mouth, and guards its 

 entrance. The mouth is in the form of a longitudinal slit, 

 and is bounded on each side by the two divisions of the hyoid 

 apparatus. Each of these consists of seventeen pieces 

 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 month ; and the 

 two posterior, after curving forward, meet at the posterior 

 inferior angle. The various pieces of which this apparatus 

 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 surrounds 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 

 occasionally 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 

 vol. i. 2 c 



