in. i INVERTEBRATE RELATIVES OF CHORDATES 49 



In all these points of development the chordates differ from the 

 annulates, but resemble the echinoderms and their allies. Further 

 features support this latter relationship. One of the most important 

 of these is that the echinoderm-like animals, and some of the early 

 chordates, have a larva with longitudinal ciliated bands, very different 

 from the trochophore larva, in which the bands run transversely 

 round the body, which is found in the other line of animals. The 

 nervous system of annulates consists of a set of ganglionated cords, 

 whereas in echinoderm-like animals it is a diffuse sheet of cells and 

 fibres below the epidermis. The nerve-cord of the chordates can be 

 derived from the latter but not easily from the former condition. 

 Many further points could be cited, for instance, the presence of a 

 mesodermal skeleton in both chordates and echinoderms, but not in 

 annulates. It may be that there are also fundamental biochemical 

 differences. Most of the spirally cleaving types of animal conduct their 

 energy transfers with arginine phosphate, whereas vertebrates, 

 amphioxus, ascidians, and ophiuroid echinoderms use creatine phos- 

 phate. Balanoglossus and echinoids have both. 



In the study of evolution it is not sufficient merely to make formal 

 comparisons, we must try to find out and compare the plan of develop- 

 ment and structure common to all members of two groups, a technique 

 often requiring great knowledge and good sense. When this is done in 

 the present case it will be found that the essential plan of development 

 of annulates involves spiral cleavage, gastrulation by immigration, 

 and a coelom formed as a schizocoele, a trochophore-like larva, and 

 full segmentation of the mesoderm. It is exceedingly unlikely that 

 such animals have given rise to chordates with their very different 

 development, which we may crudely define as showing radial cleavage, 

 gastrulation by invagination, and larva of echinoderm type. 



Extending this method we may divide the whole world of Metazoa 

 by similar criteria into Spiralia or Polymera and Irregularia or Oligo- 

 mera. The former include besides the annulates the molluscs and 

 platyhelmia, whereas the latter group contains, in addition to the 

 chordates, the echinoderms, brachiopods, polyzoa (ectoprocta), grap- 

 tolites, pogonophora, and Phoronis. The animals in this latter group 

 seem at first sight to be very different from the chordates in outward 

 form, but the farther we look into their fundamental organization, the 

 more we become convinced that the ancestors of the fish-like animals 

 are to be found here. By study of the relics of the early chordates it is 

 possible to trace the history of this strange change with some plausi- 

 bility, though its full details will probably never be known. 



