On the Brain and Skull of Amphioxus lanceolatus. 229 



more than eight myotomes (and consequeutly protovertebrae) of 

 Amphia.vas, in addition to those already mentioned, can be reckoned 

 as the equivalents ot" the ])aracliordal region of the skull in the 

 higher Vertebrates. Thus it would appear that the cranium of the 

 latter is represented by those segments of the body of AmpJdoxus 

 which lie in front of the fifteenth, counting from before backwards, 

 and that their cranial nerves are represented by the corresponding 

 anterior pairs of nerves in Amphio.rus. 



hi all Vertebrata above Amphioxus the nerves which answer 

 to the seven posterior pairs in Ampldoxus unite into one or two 

 trunks on each side, and give rise to the nerves called pneumo- 

 gastric and glosso-pharyngeal ; and as these pass out of the skull 

 in front of the occipital segment, it would appear that this seg- 

 ment is, in the main, the result of the chondritication, with or 

 without subsequent ossification, of the fourteenth protovertebra. 



There is no evidence, at present, that the ear-capsule repre- 

 sents a modification of any part of the vertebral skeleton, nor that 

 the trabecuUe are any thing but an anterior pair of visceral arches. 

 And if these parts have nothing to do with centra, or arches, of 

 vertebrte, it follows that the numerous protovertebra? which lie 

 in front of the fourteenth in Ampldoxus, are represented only 

 by muscles and nerves in the higher Vertebrata. 



The anterior end of the cerebro-spinal axis of Ampldoxus 

 answers to the lamina terminalis of the thalamencephalon of the 

 higher Vertebrata, the cerebral hemispheres and olfactory lobes 

 remaining undeveloped. 



If the auditory nerve is, as Gregenbaur has suggested, the dorsal 

 branch of a single nerve which represents both the ^jr»-</o dura 

 and the portio mollis, the auditory organ of Ampldoxus is to be 

 sought in connexion with the dorsal branch of its eighth nerve. 

 I have found nothing representing an auditory organ in this posi- 

 tion ; and I can only conclude that Ampldoxus really has no 

 auditory apparatus. In all other respects, however, it conforms 

 to the Vertebrate t\'j)e ; and, considering its resemblance to the 

 early stages of Petromyzoa described by Schultze, I can see no 

 reason for removing it from the class Pisces, But its perma- 

 nently segmented skuU and its many other peculiarities suggest 

 that it should be regarded as the type of a primary di\'ision or 

 subclass of the class Pisces, to which the name of Entotiiocrama 

 may be applied, in contrast to the rest, in which the primary 

 segmentation of the skull is lost, and which may be termed 

 Ilolocrania. On a future occasion I propose to show in what 

 manner the skull of the Marsipobranch is related to that of the 

 higher Vertebrata, and more especially to the skull of the Frog 

 in its young tadpole state. 



EXPLANATION OF THE FIGURES. 



A, C, D are diagrammatic, but a<"curate, representations of the anterior part 

 of the body in Amphioxus (A), in an Ammocate 16 inch long (C), and 

 in a fully grown Ammccaite 5*7 inches long (D). B is a copv of the 



Ann. d: Mag. X. Hist. Ser. 4. Vol. xv. 16 



