320 



COLLEGE ZOOLOGY 



probably of equal importance in the func- 

 tion of food catching. 



Circulatory system 



The circulatory system is similar to that 

 in a higher chordate such as the fish, but 

 lacks a heart. Besides the definite blood 

 vessels, there are tissue spaces into which 

 the colorless blood escapes. The subintes- 

 tinal vein collects blood loaded with nutri- 

 ment from the intestine and carries it for- 

 ward into the hepatic portal vein, and 

 thence to the liver. The hepatic vein leads 

 from the liver to the ventral aorta. Blood is 

 forced by the rhythmic contractions of the 

 ventral aorta into the afferent branchial 

 arteries, which are situated in the gill bars, 

 and then through the efferent branchial 

 arteries into the paired dorsal aortae. It 

 passes back into the median dorsal aorta, 

 and finally by way of intestinal capillaries 

 into the subintestinal vein. The blood is 

 oxygenated during its passage through the 

 gill slits. The direction of the blood flow, 

 backward in the dorsal and forward in the 

 ventral vessel, is the same as that of the 

 vertebrates, but the reverse of that in in- 

 vertebrates such as annelids. 



Coelom 



The reduced coelom is represented in the 

 adult by cavities around the digestive tract. 

 The position of the coelomic cavities is 

 shown in the pharyngeal region in Fig. 202. 



Excretory system 



The excretory organs, although often de- 

 scribed as ciliated, are actually flagellated 

 nephridia ( protonephridia ) situated near 

 the dorsal region of the phar\'nx. Each 

 nephridium bears several clusters of soleno- 

 cytes, which are flagellated cells extending 

 out of a tube in which the flagella play. 

 About 100 pairs of protonephridia connect 

 the dorsal coelom with the atrial cavity. 

 These protonephridia are not homologous 

 to the tubules of the vertebrate kidney. 



Nervous system 



The amphioxus possesses a central nerve 

 cord (Fig. 202) lying entirely above the di- 

 gestive tract in contrast to the ventral nerve 

 cords of annelids and arthropods. It rests on 

 the notochord and is almost as long. A mi- 

 nute central canal traverses its entire length 

 and widens at the anterior end to form a 

 brain vesicle, which is the only trace of a 

 brain present. An olfactory pit opens into 

 this vesicle in young specimens. At the an- 

 terior end of the nerve cord is a black pig- 

 mented spot called an "eye spot," although 

 it is not sensitive to light. The "eyes" con- 

 sist of numerous single light-sensory cells, 

 each with a pigment cup in the ventral wall 

 of the nerve cord. Two pairs of sensory 

 nerves arise from the cerebral vesicle and 

 supply the anterior region of the body. The 

 rest of the nerve cord gives off nerves on 

 opposite sides, but alternating with one an- 

 other. These nerves are of two kinds: (1) 

 dorsal nerves with a sensory function which 

 pass to the skin and ( 2 ) ventral nerves with 

 a motor function which enter the myotomes. 

 The sense organs include the olfactory pit, 

 sensory cells in the epidermis on the oral 

 and velar tentacles, "eyes," and possibly the 

 "eye spot." 



Reproduction 



In the amphioxus the sexes are separate. 

 The paired gonads (Fig. 201) project into 

 the atrium. Eggs and sperms are discharged 

 into the atrial cavity and reach the exterior 

 through the atriopore. Fertilization takes 

 place externally in the water. The cleavage 

 is holoblastic as in the starfish. 



OTHER CHORDATA 



Subphylum Hemichordata 



The hemichordates, traditionally, have 

 been considered the lowest chordates, but 

 some recent authorities do not regard them 

 as chordates at all. They say the so-called 



