Ill 



THE ORIGIN OF CHORDATES FROM FILTER 

 FEEDING ANIMALS 



1 . Invertebrate relatives of the chordates 



We have seen in the organization of amphioxus the plan of chordate 

 structure as it may have existed in Palaeozoic times. Before proceeding 

 to discuss the later forms that evolved from animals of this sort we may 

 first look yet farther backwards to discuss the origin of the whole 

 chordate phylum from still earlier ancestors. The great difficulty of 

 such an inquiry is itself a stimulus and a challenge. Typical fish-like 

 chordates were undoubtedly established by the Ordovician period, 

 but we have no good fossil record of their earliest form and this must 

 therefore be deduced from study of amphioxus and later animals. No 

 fossils that suggest chordate affinities have been found in the still 

 earlier rocks. There are, however, certain strange animals alive today 

 which, though not of fish-like type, show undoubted relationship with 

 our group. These might, of course, be degenerate offshoots from later 

 periods, but careful comparison suggests that they have been separa- 

 ted for a very long time and provide us with relics of some of the early 

 stages of our history. 



The first step in our inquiry, however, before discussing these 

 forms, should be to find out, if possible, which of the main lines of 

 invertebrate animals shows the closest affinity with the chordates. 

 Almost every phylum in the animal kingdom has been suggested, 

 including the nemertines. Many still suppose that the annelids and 

 arthropods, because of their metameric segmentation, are related to 

 the chordates, but closer examination shows that the similarities are 

 superficial. The segmentation of these annulate animals is an almost 

 complete division of the whole body into rings, and all the organ 

 systems are affected by it to some extent. In chordates only the dorsal 

 myotomal region is segmented; even the mesoderm is not divided in 

 its ventral region in most animals. Moreover, the whole orientation 

 of the body differs in the two groups. The vertebrate nerve-cord is 

 dorsal to the gut, in annulates the nerve-cord is below and the 'brain' 

 above. The blood circulates in opposite directions, the limbs are 

 based on quite different plans, and so on. Attempts have been made 

 to get over these difficulties by turning the invertebrate upside down! 

 Patten and Gaskell carried such theories to extremes and tried to 



