STRUCTURE OF THE VERTEBRATES 251 



vertebrates. Amphioxus has retained the primitive metamerism 

 and position of the nephridia, but the coelomic head differs from 

 that found in the higher invertebrates and lower vertebrates. 

 Instead of a nephrostome the head is terminated by numerous 

 minute closed knobs. The tubules are short and each opens 

 directly to the outside. 



The gonads of the vertebrates are paired mesodermal glands, 

 primitively located in the anterior part of the body, dorsal to 

 the peritoneum. The female gonad is the ovary, that of the male 

 is the testis. The sexes are separate in the majority of vertebrates, 

 an individual developing only ova or spermatozoa. The cyclo- 

 stomes and some of the teleost fishes, however, develop eggs and 

 sperms in the same animal. The condition is called hermaphrodit- 

 ism. Although isolated cases have been described in the higher 

 groups, most of the specimens are aberrant males or females 

 which have some of the characters of the opposite sex. 



A. Organs of Excretion 



The earliest vertebrate nephridia do not open independently 

 to the outside but enter a pair of pronephric ducts. The tubules 

 of the nephridia begin in the mesodermal somite at the junction 

 of the epimere and hypomere, in the region called the nephrotome 

 (see illustration on page 68). The tubules on either side grow 

 posteriorly and unite to form a pronephric duct. At the end of 

 the pronephric region the duct continues its growth toward the 

 posterior outlet. As the coelomic cavity develops the first verte- 

 brate kidney, or pronephros, makes its appearance in the ante- 

 rior part of the animal. The pronephridia are metameric and few 

 in number. Nephrostomes grow toward the coelomic cavity and 

 break through the peritoneum to enter the cavity. The two 

 pronephric ducts parallel each other to a point near the anus, 

 where they fuse to form a slight enlargement and leave the body 

 through a single opening. 



The excretion of wastes from the blood into the coelomic cavity 

 is secured by small metameric arteries. These paired renal (kid- 

 ney) arteries pass laterally and ventrally to the peritoneum 

 where they coil and form a glomendus, a knot of thin-walled 

 capillaries. The glomerulus puslies into the coelomic cavity but 



