766 CONCLUSION xxxn. i- 



oxygen was carried to the tissues in simple solution in colourless blood. 

 The circulatory system perhaps at first involved little more than an 

 irregular set of spaces among the cells, but quite early there must have 

 appeared the distinct contractile vessels, containing a blood with 

 composition distinct from that of the surrounding lymphatic or tissue 

 spaces. The method of excretion of the earliest chordates is not clearly 

 known; it perhaps involved no highly specialized cells, but occurred 

 all over the body surface. Since the animals were marine there were 

 no serious osmotic problems. Movement was by the metachronal con- 

 traction of a series of blocks of longitudinally arranged muscle-fibres 

 and this serial repetition of the muscles and their attendant nerves and 

 blood-vessels has left a large mark on the chordate plan of life. 



The nervous organization was at first based on a system of nerve- 

 cells and fibres lying spread out below the epidermis, but then became 

 concentrated dorsally in the walls of a neural tube. The special 

 receptor organs were probably simple and lay either in the skin or 

 within the tube, perhaps along its whole length, with little concentra- 

 tion at the front end and no definite anterior enlargement or brain. 

 The system functioned as a series of more or less separate reflex arcs, 

 activation coming from the stimulation of receptor organs by changes 

 in the world around. There were no large masses of nervous tissue 

 and little possibility of sustained independent action by the creatures, 

 which probably showed little flexibility of behaviour or variation of 

 action with experience. The only endocrine influences were the effects 

 of cell by-products on neighbouring tissues; there were no specialized 

 glands of internal secretion. 



Reproduction was presumably sexual (perhaps also by budding) and 

 development followed the pattern of radial (indeterminate) cleavage 

 and gastrulation by invagination, with chordo-mesoderm separating 

 from the endoderm. The young were provided with yolk for their 

 development, but were probably not otherwise cared for by their 

 parents. 



This gives a rough picture of chordate organization in the Cambrian 

 period when it probably first arose, 500 million years ago, after the 

 paedomorphic change by which a previously larval creature became 

 sexually mature. It was an organization that had already proceeded far 

 from the aggregation of similar cells that presumably characterized the 

 first metazoans. Its embryological processes were already sufficiently 

 elaborate to produce a creature with well-marked organ systems, 

 though these did not have the numerous cell types and anatomically 

 separate parts that are found later. 



