404 THE ORIGIN OF VERTEBRATES 
no time is there any evidence of any separate openings or any fusion 
with the ectoderm, such as might indicate separate openings of these 
prosomatic coxal segmental organs. Thus we see that in Limulus, 
which is presumably much nearer the annelid condition than the 
vertebrate, all evidence of separate nephric ducts opening to the 
exterior on each prosomatic segment has entirely disappeared, just as 
is the case in the metasomatic coxal glands (i.e. the pronephros) of 
the vertebrate. What is seen in the prosomatic region of Limulus, 
and doubtless also of the Eurypterids, may very probably have 
occurred in the metasomatic region of the immediate invertebrate 
ancestors of the vertebrate, and so account for the single pro- 
nephric duct belonging to a number of pronephric organs. 
The interpretation of these various embryological investigations 
may be summed up as follows :— 
1. The ancestor of the vertebrates possessed a pair of appendages 
on each segment; into the base oft each of these appendages the 
segmental excretory organ sent a diverticulum, thus forming a coxal 
gland. : 
2. Such coxal glands, even in the invertebrate stage, may have 
discharged into a common duct which opened to the exterior most 
posteriorly. 
3. Then, from some cause, the appendages were rendered useless, 
and dwindled away, leaving only the pronephric organs to indicate 
their former presence. At the end of this stage the animal possessed 
vertebrate characteristics. 
4, For the purpose of increasing mobility, of forming an efficient 
swimming instead of a crawling animal, the body-segments increased 
in number, always, as is invariably the case, by the formation of new 
ones between those already formed and the cloacal region, and so of 
necessity caused an elongation of the pronephric duct. Into this there 
now opened the ducts of the segmental organs formed by recapitula- 
tion, those, therefore, belonging to the body-segments—mesonephric— 
having nothing to do with appendages, for the latter had already 
ceased to exist functionally, and would not, therefore, be repeated with 
each meristic repetition. ’ 
This, so to speak, passive lengthening of the pronephric duct in 
consequence of the lengthening of the early vertebrate body by the 
addition of metameres, each of which contained only mesonephric 
and no pronephric tubules, is, to my mind, an example of a principle 
