386 ON THE ANATOMY OF AMPIIIOXUS LANCEOLATUS. 



ment, appear to be caused by these stria? ; and the same 

 structure probably produces similar phenomena in the apon- 

 eurosis which lines the cavity of the abdomen. 



Concluding Remarks. 



At a veiy early period in the development of every verte- 

 brated animal, the cerebro-spinal axis presents the appearance 

 of a white elongated streak. At the same period, and in 

 accordance with this simple condition of the nervous central 

 organ, the skeleton consists of a chorda dorsalis, and very soon 

 afterwards, of some of the peripheral elements of the spinal 

 column. A central organ of circulation, in the form of a tube 

 on the anterior inferior aspect of the embryo, invariably co- 

 exists 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 approaches 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 development, its position in the system might have 

 been indicated. As Anvphioxus makes a close approximation 

 to this simplicity of type, it may be useful to consider the 

 relation of its different organs one to another. 



One of the most remarkable peculiarities in the Lancelet 

 is the absence of the brain. Eetzius, 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 dis- 

 section of my own specimen, which I have also submitted to 

 the inspection of Dr. John Eeid, and of other competent 

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



