Evolution of Animals 443 



may gradually have been set aside for the double function of 

 tissue-aeration and of waste-removal, while the finer branches 

 and capillaries may have remained as excretory vessels purely. 

 The distribution of the blood-vascular system in the Nenier- 

 tinea has been carefully studied during the past sixty years, 

 though we still desiderate fuller physiological details. While 

 the simplest system (e. g., Cephalothrix) shows only two longi- 

 tudinal vessels in close proximity to the lateral nerves, in 

 higher forms there are two lateral and a dorsal vessel, which 

 with accessory vessels to the proboscis sheath, and transverse 

 ones connecting all, show a marked anticipation of the verte- 

 brate vascular system. 



In considering this system further, alike in its own dis- 

 tribution and in its aerating and excreting connections, we 

 believe it is correct to consider that proximity to the 

 nervous system for metabolic renewal of the nervous sub- 

 stance, and the periodic transfer of its blood to some aerating 

 region or regions, are of prime importance. The former is 

 effected by the frequent formation of expansions of the blood- 

 vascular system into two special longitudinal sinuses near 

 the brain, and the passage of blood-vessels parallel to the 

 nerve trunks. The latter is effected by those expansions 

 that run parallel to the proboscis sheath, or near to the cephalic 

 groove, whose vessels seem to correspond to the posterior 

 cardinals of Cyclostomata and higher forms, since in all exten- 

 sive connections are made by these vessels with the renal or 

 excretory, and with the reproductive systems. In the for- 

 ward course of these vessels to the heart, in the nemerteans 

 as in the cyclostomes, the posterior cardinals run directly be- 

 neath the sheath or notochord. 



In many genera these two vessels unite anteriorly into a 

 common cavity that seems to correspond to the venous sinus 

 in lower vertebrates, while two anterior veins that often form 

 a loop system in the front part of the head and that also unite 

 with the sinus would equally correspond to the anterior cardi- 

 nals. A single vessel in the higher nemerteans, starting pos- 

 teriorly from the anal commissure, corresponds in origin, 

 course, and position to the dorsal aorta of most vertebrates, 

 and it, along with the anterior cardinals, gives rise to the above 

 mentioned vascular loop-system round the front part of the 

 head, that agrees with the circulus cephalicus of vertebrates. 



