Evolution of Fishes from Invertebrates 87 



each vessel has an internal nucleated zone, which suggests 

 a bounding epithelial layer, but of this he is doubtful. 

 "Outside this nucleated layer there comes a thin layer of 

 very delicate fibres, doubtless muscular, arranged in a cir- 

 cular direction around the vessel. Outside the muscular 

 layer comes a single layer of large vesicular-looking irreg- 

 ularly ovoid cells, with small nuclei and slightly granular 

 contents. The wall of the vessel then, in its narrow por- 

 tions, is made up of three distinct layers." If the inner- 

 most layer should prove to be a continuous endothelium, 

 the structural agreement with average vertebrate blood- 

 vessels is perfect. 



The writer has already (7:444) summed up the mor- 

 pho-physiological conditions, as to the methods and direc- 

 tion of blood-circulation thus: 



"The distribution of the blood-vascular system in the 

 Nemertinea 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 longitudinal 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 pro- 

 boscis sheath, and transverse ones connecting all, show a 

 marked anticipation of the vertebrate vascular system. 



"In considering this system further, alike in its own 

 distribution 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 aer- 

 ating region or regions, are of prime importance. The 

 former is effected by the frequent formation of expansions 

 of the blood-vascular system into two longitudinal sinuses 

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

 to the nerve trunks. The latter is effected by those expan- 

 sions 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 extensive connections are made by these vessels 

 with the renal or excretory, and with the reproductive 

 svstems. In the forward course of these vessels to the 



