MR GOODSIR ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 257 



lation, in the form of a tube on the anterior inferior aspect of the embryo, inva- 

 riably coexists with the simplest forms of the nervous and osseous systems. 

 Branchial clefts and a liver are parts of the embryo of the vertebrated animal 

 which are never found to accompany a cerebro-spinal axis of the simplest form, 

 or a heart before it becomes divided into compartments. 



No adult vertebrated animal has hitherto been described which at all ap- 

 proaches in organisation the simplicity of the embryonic forms to which allusion 

 has just been made. Such an animal, a being perfected before the appearance of 

 branchial clefts, might have been conceived ; and, from the laws of organic deve- 

 lopment, its position in the system might have been indicated. As Amphioxus 

 makes a close approximation to this simplicity of type, it may be useful to con- 

 sider the relation of its different organs one to another. 



One of the most remarkable peculiarities in the Lancelot is the absence of the 

 brain. RETZIUS, indeed, describes the spinal marrow as terminating considerably 

 behind the anterior extremity of the chorda dorsalis, in a brain which exhibits 

 scarcely any dilatation ; but careful examination of the dissection of my own spe- 

 cimen, which I have also submitted to the inspection of Dr JOHN REID, and of 

 other competent judges, has convinced me that the spinal cord, which may be 

 traced with the greatest ease to within 1-1 Gth of an inch of the extremity of the 

 chorda dorsalis, does not dilate into a brain at all. It may be urged that we 

 ought to consider the anterior half of the middle third of the spinal marrow, where 

 it is most developed, to be the brain, and all that portion of the chorda dorsalis 

 which is in connection with the branchial cavity, as the cranium. That this does 

 not express the true relation of the parts, is evident from the fact, that this por- 

 tion of the cord, to its very extremity, gives off nerves, which are too numerous 

 to be considered as cerebral, but more especially from the mode of distribution of 

 the first and second pairs, which, in my opinion, proves the anterior pointed ex- 

 tremity to be the representative of the brain of the more highly developed verte- 

 brata. A brain of such simplicity necessarily precludes, on anatomical grounds 

 alone, the existence of organs of vision and of hearing. These special organs, de- 

 veloped in the vertebrata at least, in a direct relation with the cephalic integu- 

 ments and the brain, could not exist, even in the form of appreciable germs, in 

 the Lancelet. The black spot which RETZIUS took for the rudiment of an eye may 

 probably have been, what also deceived me at first, a portion of the black mud 

 which floats about in the branchial cavity, and which adheres obstinately to the parts 

 in the neighbourhood of the oral filaments. The first pair of nerves, although 

 very minute, in accordance with the slight development of the parts about the 

 snout, and the want of special organs of sense, might, from their position and re- 

 lations, be considered as corresponding to the trifacial in the higher vertebrata. 

 The second pair appears to be the vagus, not only from its distribution as a longi- 

 tudinal filament on each side of the body, as in other fishes, but also from its rela- 



