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 tle 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 Polycheta, 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 procelom of the vertebrate is the proceelom 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 nephroceele 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. 
