PRESIDENT'S ADDRESS. xlvii. 



vertebrates with the invertebrates in a most remarkable way, 

 linking the lower orders of fishes with the Articulata and 

 Mollusca. Its usual length is not more than two inches. The 

 head, which is scarcely distinguishable from the body, has no ex- 

 ternal indications of the existence of the organs of sense. The body 

 is marked externally by a succession of oblique striae, seen through 

 the translucent skin, and which indicate the lateral muscles. 

 Almost the only trace of the vertebral column is to be found in 

 the fibrous and cellular structures surrounding and supporting the 

 neural-axis, which is nearly uniform in size from one extremity 

 to the other, showing no enlargement at its cephalic extremity, 

 which could be denominated a brain. In the neural-arch which 

 represents the vertebrae in other animals, there is only the most 

 indistinct indication of a subdivision into segments. It is in the 

 arrangement of the organs of nutrition that the greatest approxi- 

 mation to the invertebrate type is displayed ; the mouth is lined 

 with ciliae which have a rotary motion similar to those of the 

 Rotifercc. In examining the structure of the respiratory and 

 other organs of the Atnphioxus, we are at once struck with their 

 resemblance to the Ascidian Mollusca. The movements of the 

 Amphioxus are somewhat serpentine, rapid, and powerful. It 

 passes a large part of its time either buried in the sand or 

 lying flat on its surface. In these particulars the structure of 

 the Amphioxus is similar to the higher vertebrates at the 

 very early stages of development. There is no question that 

 it must be regarded as essentially a fish. Among the inferior 

 members of the early cartilaginous group, such as the Cyclostomi 

 or circular-mouthed fish, the characteristic structure of vertebrates 

 is further developed, showing the progressive steps towards the 

 development of the vertebral-column, from the first segmentation 

 of the notochord to the complete replacement of the vertebral 

 element with the full development of the neural and haemal- 

 arches. For instance, in the case of the Sturgeon, firm carti- 

 laginous rings form the outer portion of the notochord. 



There are about eighty families of marine fishes, of which no 

 less than forty are almost universally distributed over the seas 



