612 INVERTEBRATE MORPHOLOGY. 



sally to unite with the right aortic vessel. The blood which 

 passes from the dorsal aorta to the intestine is not, however, 

 returned directly to the ventral vein, but the intestinal capil- 

 laries unite to form a vena porta which passes to the liver 

 and there breaks up into a second set of capillaries, these 

 finally emptying through the hepatic vein into the ventral 

 vessel. An hepatic portal system, resembling that found in the 

 Vertebrata, thus occurs in Amphioxus. While passing be- 

 neath the branchial region of the intestine the ventral vein 

 gives off paired vessels, the branchial arteries, opposite each 

 branchial septum, and these passing dorsalwards in the sep- 

 tum open into the dorsal aortic trunks. There is no definite 

 heart, but certain of the vessels, notably the vena porta and 

 branchial arteries, seem to be contractile. 



The notochord (Figs. 278, ch, and 279, c) has a much more 

 extensive development than in the Hemichorda, since it 

 traverses the entire length of the body. It arises from the 

 dorsal surface of the digestive tract, but early loses all con- 

 nection with the intestine ; and though in early stages it con- 

 tains traces of a lumen, this quickly disappears, the cells 

 becoming richly vacuolated, so that the uotochordal tissue 

 assumes a characteristic appearance. At either end it is 

 pointed, and throughout its entire length it is surrounded by 

 a sheath of dense connective tissue, which is continuous 

 below with the partitions separating the splauchuoccel from 

 the muscle-plates and these from one another. From each 

 side of the dorsal surface of the sheath a longitudinal lamella 

 extends dorsally, the two lamellae enclosing the central 

 nervous system and being continued above it as a strong- 

 neural ridge (Fig. 279). 



As has been already stated, the adult mouth is formed by 

 the margins of the oral hood, the original larval mouth lying 

 at the bottom of the oral cavity enclosed by the hood and 

 being surrounded by a circular fold of tissue termed the 

 velum. A short tube leads from the mouth to the branchial 

 or pharyugeal region of the digestive tract, whose w r alls are 

 here perforated by numerous slits (Fig. 278, sp) placing its 

 cavity in communication with the atrium (see Fig. 279). In 

 the adult the slits are elongated and are placed obliquely to 



