CHORD ATES IN GENERAL 157 



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 and support the 

 gills, contain the blood vessels, and are supported by chitinous rods. 



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 narroAv space surrounding the intestine. Be- 

 hind 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, 

 they break through the wall of the gonad into the atrial cavity and 

 pass out through the atriopore with the water. Fertilization occurs 

 in the water. Following fertilization comes a series of cleavage 

 divisions which are total and equal. This is followed by the infold- 

 ing of one side of the spherical body to form the gastmla and this 

 in turn becomes a free-swimming larva which reaches adult condition 

 without metamorphosis. 



The Vertebrate Animal : Subphylum Vertebrata 



In this group to which man himself belongs are found the dis- 

 tinctive chordate characteristics at some time in the life of the indi- 

 vidual. Metamerism and bilateral symmetry are universal charac- 

 teristics among vertebrates. The segmented vertebral columyi and 

 other supporting structures form an endoskeleton (internal skeleton) 

 which is the basic support of the body. Paired appendages are usu- 

 ally present at some time in the life of the individual. The majority 

 have two pairs of fins or limbs in adult condition. There is a ven- 



