258 MR GOODSIR ON THE ANATOMY OF AMPHIOXUS LANCEOLATTJS. 



tions to the hyoid apparatus and branchial cavity, to which division of organs the 

 eighth pair of fishes is specially devoted. The distribution of a branch of this 

 nerve, however, along the base of the dorsal fin, and the course of the posterior 

 part of the main branch, would appear to shew that this nerve, which I have pro- 

 visionally denominated the Vagus, is, in fact, the trifacial, which, in the higher 

 fishes, is not only distributed to all the fins, but holds exactly the same relations 

 to the dorsal and anal fins, and to the spinal nerves, as the nerve now under con- 

 sideration in the Lancelet. 



The peculiarities in the structure of the spinal cord are not less remarkable 

 than those of its configuration. It is difficult to understand, according to the re- 

 ceived opinions on the subject, how a spinal cord destitute of primitive fibres or 

 tubes, and composed altogether of isolated cells, arranged in a linear direction 

 only towards the middle of the cord, can transmit influences in any given direc- 

 tion ; and more especially how the tract of black or grey matter, if it exercises 

 any peculiar function (excito-motary) communicates with the origin of the nerves. 

 The nerves, also, are remarkable, originating in single roots, and containing in 

 their composition one kind only of primitive fibres (cylindrical). 



In reference to the skeleton of the Lancelet, it is evidently of the simplest 

 kind. If we limit the term skeleton to the Neuro-Skeleton, this animal possesses 

 only the primitive form of such a skeleton a chorda dorsalis without any cranial 

 enlargement, with a dorsal and ventral series of germs of interspinous bones and 

 fin rays peripheral elements of a spinal column. 



From a consideration of the particular class of embryonic forms to which this 

 fish corresponds, we could not expect either bone or cartilage in the composition 

 of its skeleton. Accordingly, the skeleton consists of a series of sacs, assuming 

 particular forms according to their several positions : flattened in the chorda dor- 

 salis, cylindrical in the fin bones. These sacs are easily derived, according to esta- 

 blished histological laws, from the primitive nucleated cells which constitute the 

 tissue of their representatives in the embryo, and contain, in their interior, cells, 

 or the nuclei of cells. This view of the tissue of the skeleton of the Lancelet is 

 based on a law of organization which is not usually recognized in questions like 

 the present, viz. that adult organs representing embryonic organs, are altered so 

 as to be fit for the performance of their functions, but never so far as to depart, 

 either in tissue or form, from the type of their corresponding embryonic organs. 

 The arch-shaped fin rays, described by Mr COUCH, are merely the dissepiments 

 between the cylindrical germs of the fin bones. 



The leading peculiarity of the Lancelet, considered as a representative of an 

 embryonic form in the adult series, is the want of true gills or branchial arches 

 the deficiency of branchial clefts. RETZIUS, indeed, describes an opening at the 

 posterior part of the branchial cavity, which he compares to the adominal open- 

 ings in the Myxine ; but as I have been unable to discover this opening in my spe- 



