MR GOODSIR ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 251 



neurotic laminae pass off to form septa of attachment between the muscular 

 bundles ; and along the mesial plane above the column, a similar lamina separates 

 the superior bundles of each side, and by splitting below and running into the 

 sides of the column, forms a fibrous canal for the spinal cord. Foramina exist all 

 along the sides of this canal for the passage of the nerves. A similar septum is 

 situated along the inferior part of the column, from the part where the inferior 

 muscular bundles unite at the anus, to the extremity of the tap. Along the 

 superior edge of the aponeurotic septum, between the dorsal muscular bundles, 

 and stretching from the anterior point of the vertebral column to a point beyond 

 the anus, and half embedded* between the superior extremities of the muscles, is 

 a series of closed cells of a flattened cylindrical form, adhering firmly to one an- 

 other by their bases, so as to present the appearance of a tube flattened on the 

 sides with septa at regular distances. Each of these cells is full of a transparent 

 fluid, in the centre of which is an irregular mass of semi-opaque globules, appa- 

 rently cells. This series of cylindrical sacs consists of the rudiments of inter- 

 spinous bones, and probably of fin rays, and is attached below to the fibrous inter- 

 muscular septa, half covered on each side by the lateral muscles, and enclosed 

 above by the tegumentary fold which constitutes the dorsal fin. 



A similar series of cells, with the same relations, is situated on the ventral 

 surface of the body, and stretches from the spot where the abdominal folds termi- 

 nate, to a point nearly opposite the termination of the dorsal series. 



Splanchno-skeleton, The splancho-skeleton consists of a hyoid apparatus 

 and a series of branchial ribs, seventy or eighty on each side. This division of 

 the skeleton will be described along with the respiratory apparatus, with which 

 it is intimately connected. 



Nervous System. 



The spinal cord is situated on the upper surface of the chorda dorsalis, en- 

 closed in the canal formed in the manner above described. When the whole 

 length of this canal is displayed by removing the muscles, and then carefully 

 opened, the spinal cord is seen lying in the interior, with nerves passing out from 

 it on each side. It stretches along the whole length of the spine, is acuminated 

 at both ends, and exhibits not the slightest trace of cerebral development. In its 

 middle third, where it is most developed, it has the form of a ribbon, the thick- 

 ness of which is .about one-fourth or one-fifth of its breadth ; and along this por- 

 tion, also, it presents on its upper surface a broad, but shallow groove. The other 

 two-thirds of the cord are not so flat, and are not grooved above, are smaller than 

 the middle third, and taper gradually ; the one towards the anterior, the other 

 towards the posterior extremity of the vertebral column. A streak of black pig- 

 ment runs along the middle of the upper surface of the cord. It is situated in the 

 groove already described, and is in greater abundance anteriorly and posteriorly, 



VOL. XV. PART I. 3 Y 



