20 STRUCTURE OF THE VERTEBRATES 



The capillaries draining the intestine and muscles collect into 

 veins, and these fuse to form a ventral subintestinal vein. At 

 the hepatic caecum this vein breaks into smaller vessels which 

 spread around the caecum and re-form into a continuation of 

 the ventral vein. This is the equivalent of a hepatic portal sys- 

 tem and a hepatic vein. The latter is continuous with the ven- 

 tral aorta. 



A minute group of vessels, draining the gonads and part of 

 the head region, has been described and is considered the 

 homologue of the cardinal system of vessels found in the higher 

 groups. 



Observe that the blood flows forward in the ventral vessels 

 and backward in the dorsal. This is the typical vertebrate sys- 

 tem. The student should work out for himself the relative 

 amounts of food, oxygen and waste matter contained in the 

 blood of each region of the body. 



Excretory System. The organs of excretion are paired 

 nephridia, each of which has a head in the coelomic cavity 

 and a short nephric tubule which embryologically empties to 

 the outside of the body. This is slightly modified in the adult 

 by the development of a protective atrium (see page 27). In 

 their metameric arrangement the nephridia may be considered 

 as primitive structures; but the individual nephridium is a highly 

 specialized organ. 



Reproductive System. The gonads (ovaries and testes) are 

 metameric, very numerous, and lie along the ventral half of 

 each side of the animal. As in the vertebrate the gonads arise 

 in the dorsal mesodermal tissues, outside the coelomic cavity 

 and covered by the peritoneum, the lining of the body cavity. 

 As the atrial folds develop, the gonads, each surrounded by a 

 part of the coelomic cavity and peritoneum, are pushed ven- 

 trally. Therefore, as the sperm and ova ripen they break 

 through the covering membranes and fall into the atrial cavity, 

 from which they pass to the outside through the atriopore and 

 fertilization takes place externally. This is a specialization in 

 Amphioxus, in correlation with the development of the atrium 

 (page 27), and differs from the condition found in the verte- 

 brates. In the most primitive vertebrates the ova and sperm 

 fall directly into the coelomic cavity; in all others the sperms 



