THE ANCESTRY OF THE VERTEBRATES 513 



the nerve cord in a common sheath of connective tissue. In the 

 blood system there are equal points of resemblance, for in each 

 case there are two median longitudinal vessels, one on either 

 side of the intestine. In the vertebrate the dorsal one is the 

 aorta, which sends the blood in a posterior direction, while the 

 ventral one, with a current in the reverse direction, is repre- 

 sented by the embryonic subintestinal vein posteriorly and by 

 the heart anteriorly. In an unreversed annelid it is true that the 

 dorsal blood-vessel sends its blood from tail to head, while in 

 l he ventral one the blood flows from head to tail, but by re- 

 versing the animal the correspondence in the direction of the 

 current becomes complete. The vertebrate aorta is then repre- 

 sented by the original ventral (now dorsal) vessel in which the 

 current flows backwards, and the subintestinal vein and heart 

 are represented by the original dorsal (now ventral) vessel, in 

 both cases with the current directed forwards. 



The most convincing of the many correspondences, however, 

 lies in the nephridial system of annelids and selachians, which 

 in both cases consist of segmentally arranged pairs of tubes 

 that open into the ccelom at their inner ends by ciliated nephro- 

 stomes. There are also close correspondences in the rela- 

 tion of these tubules to the germ glands and to the ccelom. 

 The more primitive type may be considered to be that found 

 in annelids, in which each somite possesses a separate ccelom, 

 or rather a pair of cceloms, since those of adjacent somites are 

 separated by transverse dissepiments, and those of the two sides 

 of the same somite by median sagittal partitions, which form 

 dorsal and ventral mesenteries. Each of the compartments 

 thus formed is supplied with a single nephrostome, the tubule 

 from which pierces the posterior dissepiment and enters the 

 next posterior ccelomic pocket, where it exhibits a convoluted 

 portion and a vesicular enlargement, and eventually opens di- 

 rectly into the exterior by a lateral opening. (Fig. 139, a and 

 b.) The germ glands develop on the anterior walls of the 

 ccelomic cavities, and the germ cells become free and float 

 about in the ccelomic fluid until they are taken up by the 

 nephrostome and find their way to the exterior, through the 



