PHYLUM CHORDATA 



373 



varies, ranging between fifty and ninety pairs. These open into the 

 atrial cavity which surrounds the pharynx and other visceral organs. 

 In the midline of the roof of the pharynx is an inverted trough, the 

 hyperhrancliial groove, which is ciliated. In the floor of the pharjTix 

 is another ciliated groove, the JiypohrancJiial groove. Its glandular 

 walls, which are capable of secreting mucus, constitute the endostyle. 

 It functions on the same plan here as in tunicates. The strings of 

 mucus entangle the food particles and are moved anteriorly, and then 

 by two peribranchial grooves are carried dorsally to the hyper- 

 branchial groove. The cilia here move the mass back to the intestine. 

 A blind, fingerlike diverticulum of the intestine, the liver or hepatic 

 caecum, extends anteriorly from its connection on the anterior part 

 of the intestine to lie on one side of the pharynx. This organ is a 

 digestive gland and empties a digestive juice containing enzj^mes into 

 the intestine. The intestine extends posteriorly to the anus as a rela- 

 tively straight tube. The food is digested in, and absorbed from, the 

 intestine. 



Respiratory System and Respiration. — As stated above, the water 

 in passing through the gill slits delivers oxygen to the blood in the 

 capillaries there and absorbs carbon dioxide from it. The water 

 then passes back through the atrial cavity and out through the 

 atriopore. The blood then distributes the oxygen to all tissues of 

 the body. The gill-'bars, which separate the slits, contain the blood 

 vessels, and are supported by chitinous rods. The gills are on the 

 faces of the gill bars and are covered with cilia which help move 

 the water through its course. 



Excretory System and Excretion. — Ciliated nephridia similar to 

 those of the earthworm lead from the dorsal portion of the coelom 

 to the atrial cavity. The coelomic cavity is reduced in the pharyn- 

 geal region to a narrow space surrounding the dorsal aorta above 

 the pharynx and a narrower one around the ventral aorta below. 

 Between the posterior end of the pharynx and the atriopore, the 

 coelom consists of a narrow space surrounding the intestine with 

 a thin membrane separating it from the atrial cavity. Behind the 

 atriopore it is relatively larger. 



Reproductive System and Life Cycle. — This animal is dioecious 

 with each mature individual possessing 26 pairs of (31 to 33 pairs 

 in B. calif orniense) nodular gonads embedded in the body wall near 

 the base of the metapleural folds. When the germ cells mature, 



