432 THE ORIGIN OF VERTEBRATES 



3. The metasomatic appendages disappeared owing- to their enclosure by 

 pleural folds, which., meeting- in the mid-ventral line, not only caused the 

 obliteration of the appendages, and gave a smooth fish-like body-surface to the 

 animal, but also caused the formation of an atrial cavity. 



4. Into these pleural folds the dorsal longitudinal muscles of the body 

 extended, and ultimately reached to the ventral surface, thus forming the 

 somatic muscles of the vertebrate body. 



5. When the pleural folds had met in the mid-ventral line the animal had 

 became a vertebrate, and was dependent for its locomotion on the movements 

 of these somatic muscles, and not on the movements of appendages. Conse- 

 quently, elongation of the trunk-region took place, for the purpose of increasing 

 mobility, by the formation of new metameres. 



6. Each of such metameres possessed its own segmental excretory organ, 

 formed in the same way as the previous pronephric organs, but, as there were 

 no appendages in these new-formed segments, the excretory organs took on the 

 characters of a mesonephros, not a pronephros, and opened into the pronephric 

 duct, because the direct way to the exterior was blocked by the enveloping 

 pleural folds. 



7. The group of annelids from which the protostracan ancestor of the 

 vertebrates arose was the highest annelidan group, viz. the Polychada. as 

 shown by the nature of the excretory organs in Amphioxus. 



8. The coxal glands of the protostracan ancestor existed on all the segments, 

 and were, therefore, divisible into three groups, prosomatic, mesosomatic. and 

 metasomatic ; these three groups of coxal glands still exist in the vertebrate 

 as ductless glands. 



9. The prosomatic coxal glands form the pituitary body. 



10. The mesosomatic coxal glands form the thymus, thyroid, parathyroids, 

 tonsils, etc. 



11. The metasomatic coxal glands form the adrenals. 



12. The proccelom of the vertebrate is the proccelom of the protostracan 

 ancestor, which splits into a dorsal part, the myoccele, and a ventral part, the 

 nephrocele. This latter part not only forms the pronephros and mesonephros, 

 but also by a ventral extension gives origin to the walls of the vertebrate body- 

 cavity or metaccele. 



13. This ventral extension of the original nephroccele at first excreted to 

 the exterior, through abdominal pores, or through peritoneal funnels. When 

 such paths to the exterior became closed, it also became a ductless gland, 

 belonging to the lymphatic system. 



