VASCULAR SYSTEM OF VEEMES. 



167 



and the lateral ones at regular distances. In this way the whole 

 arrangement is in a way segmented, and corresponds to the meta- 

 merism indicated in other organs. 



The canal system which ramifies through the integument of the 

 Acanthocephali, and is also united with the canals of the lemnisci 

 (p. 174), must not be regarded as a similar system 

 of vessels. It is uncertain to what morphological 

 group of organs they belong. 



§ 138. 



The vascular system of the Annulata resembles 

 that of the Nemertina in all essential points. In 

 almost all there are longitudinal trunks, which 

 have a dorsal and ventral, or even a lateral, 

 course ; these are connected with one another by 

 transverse anastomoses, and pass into one another 

 in the anterior as well as in the posterior region. 

 The dorsal longitudinal vessel, which runs above 

 the enteron, is the most constant in character; 

 it is always contractile, and the current of blood 

 in it is driven from behind forwards. It cor- 

 responds to the dorsal median vessel of the Nemer- 

 tina, while the two lateral vessels of the latter 

 may correspond to the ventral vessel of the Annu- 

 lata. These vessels are not closed in all Annulata, 

 but are connected with wider spaces, which repre- 

 sent a body-cavity. The vascular system is not 

 completely differentiated in these forms. The 

 body-cavity remains in direct connection with 

 the vascular system in the Hirudinea, owing to 

 the fact that organs which formerly lay in the 

 ccelom are now enclosed in hasmal spaces. There 

 are usually three such sinuses. A median one, 

 representing the principal portion of the coeloni, 

 embraces, in Clepsine and Piscicola, the alimentary 

 canal and the ventral medulla, and perhaps also 

 a portion of the dorsal vessel, except where this 

 has, as in Piscicola, a special sinus for itself. 

 Two pulsating lateral vessels (Fig. 61, B I) partly 

 communicate with the median sinus, and are 

 partly connected to each other by transverse 

 anastomoses. In Hirudo and its allies, the median sinus is, at first, 

 only present in the region of the head, where it surrounds the oeso- 

 phageal ring. It has only a ventral development in the rest of 

 the body, where it encloses the ventral medulla (Fig. 61, B n). 

 This disappearance of the great sinus is due to the development 

 of a fine vascular network in .its place, and, like it, is connected 

 with the transverse vessels, which unite the longitudinal vessels with 



Fig. 76. Diagram of 

 the vascular system 

 of the Nemertina. 

 d Dorsal longitudinal 

 trunk. 11 Lateral 

 vessels. The arrows 

 indicate the direction 

 of the stream of 

 blood. 



