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 of 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 



