48 ORIGIN OF CHORDATES in. i 



show a relationship of chordates with the eurypterids, heavily ar- 

 moured arachnids of the Cambrian and Silurian. These animals show 

 a certain superficial resemblance to some early fossil fishes, the cepha- 

 laspids of the Devonian (Fig. 83), and these workers, with great 

 ingenuity, claimed to find in them evidence of the presence of many 

 chordate organs. 



The safest evidence of affinity is a similarity of developmental pro- 

 cesses : animals that develop very differently are unlikely to be closely 

 related. The development of modern annulates is utterly different 

 from that of chordates. The cleavage by which the fertilized egg is 

 divided into blastomeres follows in annulates a 'spiral' plan, in which 

 every blastomere arises in a regular way and the future fate of each 

 can be exactly stated. In later annulates, such as the arthropods, this 

 plan is complicated by the presence of much yolk, but even in these 

 animals the cleavage does not resemble that of chordates, which is 

 radial or 'irregular', the cells not forming any special pattern. This 

 characteristic has been used to divide the whole animal kingdom into 

 two major groups, Spiralia and Irregularia. 



The next stage of development, gastrulation, by which the ball of 

 cells is converted into a two-layered creature, also occurs very differ- 

 ently in the two groups. Our knowledge of the mechanics of the pro- 

 cesses by which this change is produced is still imperfect, in spite of 

 recent advances, but in lower chordates it occurs by invagination, the 

 folding in of one side of the ball of cells to form an archenteric cavity 

 communicating with the exterior. In annulates this is never seen; 

 the cells that will go to form the gut migrate inwards either at one 

 pole or all round the sphere and only later form themselves into a 

 tube, which comes to open secondarily to the outside. It is probable 

 that when we know more of the forces by which the gastrulation is 

 produced the difference will appear even more marked than it does 

 from this crude and formal statement that gastrulation in chordates is 

 by invagination, in annulates by immigration. 



The same applies to the method by which the mesoderm and coelom 

 are formed. In lower chordates the third layer is produced by separa- 

 tion from the endoderm, so that the coelom is continuous with the 

 archenteron and is said to be an enterocoele. In annulates cells 

 separate in various ways to form the mesoderm and a coelom then 

 arises within this solid mass as a schizocoele. It is true that in some, 

 indeed many, of the higher chordates the coelom is never continuous 

 with the archenteron, but its method of development shows it to be 

 a modified enterocoele. 



