212 



Introduction to Animal Morphology. 



Fio;. 28. 



little concretions may exist. The anus is terminal, or 

 dorsally sub-terminal. 



The vascular system may be absent (Glyceria, 

 Tomopteris,* &c.) or rudimental. Polygordius has a 

 single dorsal vessel giving off csecal transverse 

 branches. Most worms have four longitudinal canals — 

 I, a pulsating dorsal vessel, double in 

 Nephthys, carrying blood from behind 

 forwards, lying in a cellular mass on 

 the digestive canal ; 2, a ventral vessel, 

 rarely pulsating (Clymene, Maldania), 

 or double (Eunice)t ; 3, two lateral 

 symmetrical trunks, usually accom- 

 panying the lateral nerves, rarely pul- 

 sating (Protula Dysteri). These stems 

 are joined by terminal and metameric 

 cross branches, some of which may be di- 

 lated, acting as simple hearts (Ssenurus, 

 &c., Fig, 28) ; these give off parietal and 

 visceral branches, some of which end 

 caecally, but most others have a closed 

 or lacunary communication between 

 the arterial and venous branches. To 

 these branches contractile caeca may be appended 

 (Lumbriculus variegatus), which may lie in contact 

 with the ducts of the segmental organs. In the cepha- 

 lobranchiate forms the dorsal vessel dilates into a pul- 

 sating sac (branchial heart) on the pharynx, sending 

 off gill-vessels, which return to the ventral stem. In 

 Polyophthalmus the dorsal vessel divides into lacunary 



Circulatorj- vessels 

 of S;enuris variegata, 

 showing the dorsal 

 and ventral vessels, 

 the transverse anas- 

 tomoses, and the 

 cardiac dilatations of 

 one of the latter. 



* In these the perienteric coi-pusculated fluid circulates in the body 

 cavity, moved by the peritoneal cilia. 



t There may be two dorsal and two ventral vessels in Hermella. 



