50i CHORD AT A. 



541, 543, 1). The Avascular system, with colorless blood, consists 

 of a dorsal arterial («) and a ventral venous trunk connected by 

 lateral loops or arches. The ventral trunk begins as a subintestinal 

 vein under the intestine, branches as a portal vein over the liver, 

 and, reuniting again in a ventral vessel, continues forward, as the 

 aorta ascendens, below the gills. From this the vascular arches — 

 gill arteries — pass uj) between the gill slits and form the dorsal 

 vessel, the aorta descendens. A true heart is lacking, but various 

 parts of the vessels — a part of the ventral trunk and the bases of 

 the gill arteries — are contractile, whence the name Leptocardii. 



As the pharynx lies in the peribranchial chamber, the digestive 

 portion of the tract lies in a true body cavity or cwlom, which ex- 

 lends forward (fig. 542, co) into the branchial region as well as 

 into the gill-walls (branchial ccelom) and into the outer walls of 

 the peribranchial chamber (jjeribranchial C(xdom). In the jjeri- 

 branchial ccelom are the gonads [g), a series of 23oueh-like cell fol- 

 licles which, by dehiscence, allow their joroducts to escape into the 

 peribranchial chamber. Into this chamber also empty the excre- 

 tory organs which were long sought for in vain. These are [n) a 

 series, on right and left sides, of ciliated canals apparently cor- 

 responding to the pronephros of the vertebrates. Each canal 

 begins with at least one ciliated nephrostome in the ccelom and 

 opens separately like an annelid nephridium. 



Like the structure, the development is comparatively simple. The 

 following points deserve special mention : (1) The eggs have a nearly 

 equal segmentation (fig. 96). (2) A typical invaginate gastrula (fig. 105) 

 occurs. (3) The mesoderm arises as a series of pouches, right and left, 

 from the mesenteron, which later separate and represent the primitive 

 segments. Hence these are clearly mesothelial in nature. From the cavi- 

 ties of these arises the body cavity, which is consequently an enterocoele. 

 (4) The dorsal surface of the entoderm between these coelomic pouches 

 becomes folded off from the rest and forms the notochord, which Ues 

 between the digestive tract and the nervous system. (5) The nervous 

 system arises from a longitudinal groove which becomes folded into a 

 tube and is connected for a while with the digestive tract by a neureu- 

 terio canal. 



Ainpliioxus* contains a few closely related species which occur on our 

 southeastern coast, in Europe, Indian Ocean. Recently other genera have 

 been Ae&mheA—Asymmetron * in America, Heteropleuron in the South 

 Seas. The animals live in quiet bays and bury themselves in the sand, 

 with only the mouth above the surface. Like all animals with rudimen- 

 tary eyes, they shun the liglit and are greatly excited by strong illumi- 

 nation. 



