The vascular or so-called pseud-haemal system is absent in some Polychaeta, i. e. Glyceridae, Capitellidae, Polycirrus, Tomopteris, which may hence be termed An-angian1 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 Stanrocephalns, 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 oesophageal 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, etc. 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 modifications in the anterior ends of the longitudinal trunks.
1 The two outgrowths of the oesophagus in Nereidae are possibly also air-bladders. The intestine of Phyllodoce lamelligera has been found to contain air. The four families, Hesionidae, Syllidae, Nereidae, Phyllodocidae possess no branchiae. See Eisig, Mitth. Zool. Stat. Naples, ii. 1881, pp. 293-4.
2 The epithelial cells of the hinder part of the intestine in some Syllidians contain refractile spherules in large numbers. Claparfede, supposing them to be of excretory character, termed this portion of the intestine 'region urinaire.'
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 haemoglobin dissolved in the plasma2, or green as in the Chlorhaemidae ( = Pherusidae), e.g. Siphonostoma, in Spirographis Spallauzanii ( = Sabella ventilabrum), and in many Serpulinae, owing to the presence of a respiratory pigment, chlorocruorin, capable of oxygenation and de-oxygenation. Colourless corpuscles of fixed outline are found in the fluid (most Oligochaeta, several Polychaeta, i. e. Eunice, Staurocephahts, Nereis, Syllidae, Ophelia, Cirrhatulus), pinkish corpuscles in Magelona (Mcintosh). Special corpuscles tinted with haemoglobin are met with in the coelomic fluid of the anangian Glycera and Capitella, and red-coloured corpuscles occur similarly in Heteroterebella sanguinea which possesses a pseud-haemal system (Claparede). Amoeboid corpuscles are always present in the coelome.
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 Aphrodite aculeata, in less quantities in the muscular tissue of its pharynx (P. R. S. xxi. 1879, p. 75).
Respiration is carried on partly by integumental capillaries, when present, and in many Polychaeta by branchiae (p. 595).
The excretory organs or nephridia occur as a rule one pair in each somite, but they are generally aborted in the anterior region of the body, especially when there is a muscular pharynx. Their number is much reduced in some Tubicola, e. g. to eight pairs in Terebella gigantea, six pairs in Arenicola, and they are therefore present in certain somites only which may be anterior as in the two genera just named, or median and posterior as in Chaetopterus, Sabella, Myxicola. Each organ consists essentially of a ciliated tube terminating internally in a ciliated and generally funnel-shaped orifice, and opening externally by a nephridiopore. This pore varies in position. It is in most Oligochaeta near the ventral bundle of setae (see p. 205); near the parapodium in Polychaeta, either ventral to it, e. g. in Polynoe, or dorsal, e. g. Sthenelais, Notomastus, or in front of it The section of the tube contiguous to the pore may be wider in calibre as in Lumbricus and its allies, or vesicular as in aquatic Oligochaeta and some Tubicola, and in both cases is provided with a muscular coat 1 The vesicle may have a glandular epithelium.
It may be the only persistent part of the organ, e. g. in the anterior somites of the tubicolous Ophelia, or it maybe entirely wanting as in the posterior somites of the same animal. So far as is known it is not represented in the Errantia, nor in some Tubicola, e. g. Hermella. The remaining part of the nephridium is usually short in Polychaeta, but long in Oligochaeta, and disposed in loops (p. 204). The whole organ is either contained in a single somite, as in most Polychaeta, or the funnel opens into a somite anterior to that in which lie the ciliated tube and external aperture as in Oligochaeta, the AIciopidae among Errantia (Claparede), and the larval nephridia of Capitella (Eisig). The nephridia sometimes enlarge at the reproductive season, as in Spio Mecznikowianus and the female somites of the hermaphrodite Microphthalmus. Certain important variations have been observed. The nephridial funnel is absent in the Oligochaete family Chaetogastridae. The number of nephridia in a somite is increased in the terrestrial Oligochaete Acanthodrilns multiporus (p. 204) and in the Capitellidae. Notomastus in this family has occasionally more than one pair in a somite; Capitella capitata on the contrary has always 3-4 pairs in the anterior somites in which they occur, 4-5 in the median, 5-6 in the posterior.