52 ZOOLOGY sect. 



poured into a median longitudinal vessel, the sub -intestinal vein 

 (Figs. 742, B, and 745, s. int. v.), lying beneath the intestine : in this 

 trunk the blood flow^ forwards, and, at the origin of the liver, passes 

 insensibly into a hepatic portal vein (hep. port, v.), which extends 

 along the ventral side of the liver and breaks up into capillaries in 

 that organ. From the liver the blood makes its way into a. hepatic 

 vein (hep. v.), which extends along the dorsal aspect of the digestive 

 gland, and, turning downwards and forwards, joins the posterior 

 end of the ventral aorta. 



It will be seen that the vascular system of Amphioxus consists 

 essentially of (a) a dorsal vessel represented by the paired and 

 unpaired dorsal aortse, (6) a ventral vessel represented by the 

 sub-intestinal vein and the ventral aorta, and (c) commissural 

 vessels represented by the afferent and efferent branchial arteries 

 and the intestinal capillaries. So far the resemblance to the 

 vascular system of Annulata is tolerably close ; but two important 

 differences are to be noted. The blood in the ventral vessel 

 travels forwards, that in the dorsal vessel backwards — the precise 

 opposite of what occurs in Worms, and the ventral vessel is broken 

 up, as it were, into two parts, by the interposition in its course of 

 the capillaries of the liver, so that all the blood from the intestine 

 has to pass through that organ before reaching the ventral aorta. 

 This passage of the intestinal blood through the vessels of the 

 liver constitutes what is called the hepatic portal system, and is 

 eminently characteristic of Vertebrata. 



The Uood is colourless, and appears to contain no leucocytes. 

 It is not confined to the true blood-vessels just described, but 

 occurs also in certain cavities or lymph-spaces, the most important of 

 which are the cavities in the dorsal and ventral fins containing the 

 fin-rays (Fig. 744, fh.), and paired canals in the metapleures {of.). 



Excretory Organs. — The principal organs of excretion are 

 about ninety pairs of peculiarly modified nephridia (Fig. 743, neph.) 

 situated above the pharynx and in relation with the main 

 coelomic cavities. Each nephridium (Fig. 746) is a bent tube 

 consisting of an anterior vertical and a posterior horizontal limb. 

 The vertical limb terminates in a large group of solenocytes (Vol. 

 I., p. 479), and there are several smaller groups on the horizontal 

 limb. The organ thus closely corresponds to the type of nephridium 

 with closed inner end bearing solenocytes already described as 

 occurring in certain of the Polychseta. On the ventral surface 

 of the horizontal limb, opposite a secondary branchial lamella, 

 is a single aperture bearing long cilia and opening into the 

 atrium : this corresponds with the nep>hridiopore or external 

 aperture of the typical nephridium. 



An excretory function has also been assigned to a single pair 

 of organs called the hroivn funnels (Fig. 743, br. /.), also situated on 

 the dorsal aspect of the pharynx at its posterior end. Their 



