AMPHIOXUS. 



obliquely directed rods, and form over the rods 

 leaf-shaped, inwardly projecting branchial folds. 

 Between the latter there are slit-like openings 

 for the outflow of the water, which passes into 

 a superficial cavity the atrial cavity produced 

 secondarily by the growing over of a fold of the 

 integument and opening to the exterior by a 

 pore the atrial pore on the ventral side. 

 The intestine begins at the posterior end of 

 this branchio-pharyngeal sac, and passes in a 

 straight course as far as the tail, where it opens 

 by a somewhat laterally-placed anus. The in- 

 testinal tube is divided into two regions, of which 

 the anterior receives a csecal hepatic sac, which 

 extends forwards on the left side of the pharynx. 

 The vascular system is without an in- 

 dependent heart, but in its place the principal 

 vessels pulsate. The arrangement of the vessels 

 permits of comparison with the vascular ap- 

 paratus of the Invertebrata (Annelids), and at 

 the same time it represents, in the simplest 

 form, the arrangement typical of Vertebrates. 

 A longitudinal trunk running beneath the 

 respiratory sac gives off numerous vessels, which 

 are contractile at their origin, to the gills. The 

 anterior pair of these branchial arteries forms a 

 contractile vascular arch placed behind the 

 mouth, the two parts of which unite beneath 

 the notochord to form the aorta, which receives 

 the next following branchial arteries. The 

 venous blood returning from the organs is 

 collected in a vessel placed above the hepatic 

 caecum ; this vessel becomes the subpharyngeal 

 longitudinal trunk. The blood returned from 

 the intestinal canal is collected in a vessel the 

 portal vein which breaks up into fine branches 

 on the hepatic caecum. A second contractile 

 vessel (vena cava) receives the blood from these 

 branches, and conducts it back into the sub- 

 pharyngeal longitudinal trunk. The .blood 

 corpuscles are colourless. 



151 



