THE PHYLOGENESIS OF VERTEBRATES 27 



shore water of the warmer seas, usually buried in the sand in 

 a perpendicular position, with the anterior end projecting into 

 the water, expanded into a sort of hood for the collection of 

 its food. When fully grown it is about two inches in length 

 and is in the form of a cylinder, flattened laterally, and 

 pointed at either end. It is divided into a succession of body 

 segments, somites, by V-shaped lines, which represent the 

 edges of the partitions of connective tissue, the myocommata. 

 These run through the masses of body muscles, and divide 

 them into segmental portions, the myomeres. The internal 

 skeletal axis, which forms one of the chief characteristics of 

 the group of vertebrates, is here represented by a flexible 

 cylindrical rod of a substance resembling cartilage, running 

 through the body from tip to tip. This rod, the notochord, 

 shows no trace of segmentation, and it is thus seen, as is 

 also the case in all vertebrate embryos, that the segmentation 

 so fundamentally characteristic of vertebrates, and so well 

 marked in their internal skeleton (vertebrae, ribs, etc.), was 

 acquired first by the muscular system, perhaps as an adapta- 

 tion to facilitate the flexibility of the body, and that it was 

 secondarily carried over to the skeleton. 



In arranging a phylogenetic tree of the vertebrates, Amphi- 

 o.nis should be placed at the bottom, although, if absolute 

 accuracy is demanded, neither Amphioxus nor any modern 

 animal, with its later modifications, should be placed at any 

 point along the main stems of the phylogenetic tree, but all 

 should be placed at the termini of branches ; proximity to the 

 ancestral line being indicated by the shortness of the branch. 



If, however, later modifications, since they have undoubt- 

 edly affected all modern forms to a greater or less extent, 

 may be left out of account, and if the successive animal forms 

 may be placed in the positions occupied by their direct ances- 

 tors, we may thus form a phylogenetic tree like the one given 

 here, which expresses the relationships of modern forms to 

 one another in a simple and essentially correct manner. 



Above Amphioxus ensues a great gap, the greatest in the 

 entire series, bridged over by no forms, either living or fos- 



