CHAETOPODA. 601 



hence be termed An-angian l . It constitutes in all others a closed system 

 of tubes containing a fluid apparently respiratory in function. The system 

 of tubes is divisible into main longitudinal trunks, branches connecting 

 them inter se, and capillary networks distributed to the various organs. 

 The principal longitudinal vessels are two, a dorsal or supra-intestinal, 

 and a ventral or supra-nervian, to which may be added a sub-intestinal 

 apposed to the ventral aspect of the intestine. A pair of lateral vessels 

 may extend along the body-walls for a greater or less distance ; another 

 pair may accompany the nerve cord, one vessel on either side ; and in some 

 terrestrial Oligochaeta there is a sub-nervian vessel. The dorsal vessel is 

 double in development and occasionally retains its double character for a 

 greater or less extent, e.g. in Eunicidae, in Staurocephahts, Pectinaria, 

 Acanthodrilus sp. ? The ventral vessel is said to be double in Nephthys, 

 Myxicola, Sabella : both vessels in Hermella.* The dorsal vessel is usually 

 connected to a capillary network in the walls of the intestine which is 

 replaced in some Tubicola (Serpidinae, Sabellinae,Ariciidae,Ammocharidae, 

 Chaetopteridae) by a contractile blood-sinus, the dorsal vessel being itself 

 suppressed in this region. It is connected to the supra-nervian vessel by 

 direct vascular loops in the anterior region of the body, and sometimes for 

 the greater part of its length by the intestinal capillaries, by integumental 

 capillaries in most Polychaeta and terrestrial Oligochaeta. The pseud- 

 haemal fluid usually flows forwards in the dorsal, backwards in the ventral 

 vessel, but the direction may be temporarily reversed, or even permanently, 

 as in Pectinaria neapolitana (Claparede). The flow is maintained by 

 contractions of the dorsal, more rarely of the ventral vessel, or by specially 

 enlarged vessels, either a portion of the dorsal vessel, e.g. in the oeso- 

 phageal region of Terebellidae, or one or more pair of lateral loops 

 connecting the dorsal and ventral vessels, e. g. in many Oligochaeta, in 

 Ariciidae and Chaetopteridae, &c. Many Serpulidae have a voluminous 

 contractile plexus in the oesophageal region to which the intestinal sinus 

 and ventral vessel are severally connected, and from which the branchial 

 vessels take origin. The presence of a protrusible pharynx causes modifi- 

 cations in the anterior ends of the longitudinal trunks. 



The vessels are composed in some, probably in all, instances of an 

 epithelial coat, to which may be added, in the larger vessels, a muscular 

 coat. The contained fluid is nearly colourless in Aphrodite, feebly yellow 

 in Chaetopterus, red in most Oligochaeta and some Polychaeta, e. g. Eunice, 

 Nereis, Arenicola, Cirrhatulus, Terebella, the colour being due to haemo- 

 globin dissolved in the plasma 2 , or green as in the Chlorhaemidae 



1 It is often said to be absent in Aphroditidae, but has been detected in Aphrodite itself and 

 some other members of the family. The almost colourless character of the contained fluid is the 

 cause of its invisibility here. 



2 Haemoglobin is said by Lankester to occur in large quantities in the nervous system of Aphro- 

 dite acuhata, in less quantities in the muscular tissue of its pharynx (P. R. S. xxi. 1879, p. 75). 



