Evolution of Fishes from Invertebrates 89 



sal aorta. A corresponding series of somatic veins also 

 empty into the cardinals (laterals). 



"As to the methods and direction of blood circulation 

 our knowledge is still vague. Burger considers that the 

 blood flows along the dorsal aorta which is the most power- 

 ful pulsating one. It then flows into the transverse vessels, 

 thence into the lateral vessels, from which it is propelled 

 forward. This, if fully proved to be correct, would agree 

 fairly well with the cyclostome circulation. Interesting 

 also is Bohmig's discovery in nemerteans of unicellular 

 valvular swellings that occur at irregular intervals along the 

 interior of the vessels, and there act as valves for the course 

 of the blood flow. 



"The nucleated and often pigmented blood corpuscles 

 contain true haemoglobin in not a few cases, both of which 

 are characters that lead up to the vertebrates." 



XII. The excretory or nephridial system. 



In passing from the blood-vascular to the excretory 

 system, a highly important question presents itself. For 

 amongst the simpler types of invertebrate, no recognisable 

 blood-system is observable, though an excretory one exists 

 from infusors upward. The first beginnings then of the 

 former deserve to be traced. The writer, in regarding 

 the rhabdocoel turbellarians as ancestral to nemerteans, 

 suggested the following for them: "in view of the ex- 

 treme and varied ramifications of the excretory system in 

 'rhabdocoels' as described and figured by von Graff, in 

 view also of what is noted below regarding the joint blood- 

 vascular and excretory systems of nemerteans, it seems not 

 unfair to suggest that, in transition from the rhabdocoels 

 to the nemerteans, a portion and specially certain of the 

 main trunks of the excretory system in the former, may 

 gradually have been set aside for the double function of 

 tissue aeration and waste-removal, while the finer branches 

 and capillaries may have remained as excretory vessels 

 purely." He then went into details that favored such. 



The writer was not then aware that Dendy and Spen- 

 cer had both suggested, some years before, a possible tran- 

 sition relation between excretory tubes and circulatory 



