A vascular system is developed in Enteropneusta, most Chaetopoda, the Polygordiidae among Archi-Annelida and the Gephyrea, with the exception of Priaptdidae1. It consists of a system of tubes, some portion of which is contractile, completely closed off from the coelome. In Hirudinea the vessels appear to communicate with the remnants of the coelome as well as with the secondarily formed metacoelome. Haemoglobin occurs in the vascular plasma of many Chaetopoda, of Gnathobdellidae among Hirudinea, in the blood corpuscles of some Nemertea. Another respiratory pigment, chlorocruorin, is found in some Chaetopoda. Specialised haemoglobin-tinted corpuscles are found in the coelome of a few Chaetopoda, and the Gephyrean Thalassema Neptuni: haemerythrin replaces the haemoglobin in some members of the Gephyrean family Sipimctdidae. Colourless corpuscles of fixed outline occur in the vascular system of many Chaetopoda, some Gephyreans, as well as in the coelome of some of the latter (Priapirtidae).
1 The Nemertea also possess a vascular system. The cephalic vessels are spoken of by Oudemans as lacunar, and Hubrecht regards the whole system as well as the cavity of the proboscis-sheath as remnants of an archicoele. The communication in some Nemerteans between the nephridia and the vascular lacunae is in favour of this view.
No excretory or nephridial system has been discovered in Chaeto-gnatha and Acanthocephala, and, if present in Enteropneusta, it has a peculiar form, unless the canals communicating with the coelomic cavities of the collar and proboscis may be deemed to be metamorphosed nephridia. It is peculiar also in Nemertea and Nematoda. But in other Vermes it falls under one of two types, according as to whether it commences (I) by flame-cells, or (2) by ciliated funnels. A flame-cell is a cell bearing a long flattened but pointed cilium, which projects freely into a funnel-shaped space with proper walls. The cell may or may not have basal processes connecting it to the surrounding tissues. The funnel-shaped space is continuous at its apex with a tubular canalicule, by which it is connected to the system of excretory vessels. It is said by Fraipont to have, in certain instances at least, e.g. in Cestoda, a lateral opening into the coelomic spaces of the mesoderm; but the existence of this opening is denied by Pintner and others. Such flame-cells are found in Trematoda, Cestoda, Turbellaria, and Rotifera. The system of excretory vessels with which they are connected varies in disposition to a certain extent.
It may have, as in some Cestoda and Ttirbellaria, numerous external apertures, which show a more or less perfect segmental arrangement in certain of the latter (some Triclads); but as a rule there are only one or two apertures variously situated and terminal to the main canals. The vessels themselves seem to be intracellular, i.e. to consist of a series of perforated cells, and cilia may occur in their course. They may form a network, but in most cases there are two or more main longitudinal canals, the terminations of which may be contractile. The second type of excretory system occurs in Chaetopoda, Archi-Annelida, Hirndinea, and in Gephyrea in a modified form. It begins with a series of intercellular funnel-shaped apertures, composed each of a variable number of cells bearing cilia. The funnels have primitively a segmental, arrangement, a pair to each somite of at least the middle region of the body, but this arrangement may be lost or masked. They lead in Chaetopoda and Archi-Annelida into tubes which may be in part intracellular, are sometimes convoluted, and have segmentally disposed external apertures.
In the Hirudinea the excretory tubes are also intracellular, but though their external apertures are segmentally disposed, they form a network which is either continuous or has become much restricted and discontinuous 1. Two sets of nephridia are distinguishable in Gephyrea except Priapididae, where no excretory apparatus has been detected: (I) sacs, each with a single funnel leading into the coelome, typically paired but restricted at the outside to four pairs in all, and opening externally on the ventral aspect; (2) sacs, with numerous funnels leading into the coelome, and opening externally with the rectum; but there is reason to believe that this communication with the rectum is secondarily acquired1. The second kind of nephridia (2 supra) is present only in Gephyrea Chaetifera, side by side however with the first kind. The type of renal organ with ciliated funnels is limited to groups in which a coelome is present, and is probably an adaptation to it (Lang); and it is a noteworthy fact that the funnels so far as is known are developed independently of their excretory tubes. The Rotifera are said to be an exception to the statement, in so far that a coelome is present, yet their nephridia belong to the first type.
The same is true of some Turbellaria. Their coelome however is probably an archicoele. Larval or provisional nephridia, commencing with flame-cells, occur in many Polychaetan Trochospheres; with the flame-cells aborted in Polygordius among Archi-Annelida, and in Echiurus among Gephyrea, and in a very rudimentary condition in Hirudinea. A pair of these organs is found in the head, and may be branched as in Polygordius and Echiurus. A second pair of organs in Polygordius has a structure similar to the first pair. A right and left longitudinal canal grows backwards from the first pair or cephalic nephridia in Polygordius, and gives origin to the permanent nephridia, afterwards disappearing. Similar but persistent canals connecting the permanent nephridia have been discovered in Polymnia nebulosa ( = Terebella Meckelii). In the Polychaetan Trochospheres a pair of these larval nephridia is found in the head, and one or more pairs in the body. Provisional cephalic nephridia have also been observed in larval Oligochaeta. It is possible that the excretory system of the Turbellaria, especially the Triclada, with its more or less segmentally arranged apertures as well as longitudinal canals, represents a primitive condition which becomes modified in other cases (1) by the suppression of the longitudinal canals, and (2) by the formation of intercellular funnels coupled with a well-marked segmental arrangement of the parts persisting and of the external apertures2. The relation which the nephridia acquire, either temporarily or permanently, to the sexual organs is entirely secondary.
1 The presence of nephridial lobes in the medicinal Leech and its allies is" perhaps best explained on the hypothesis that the lobes represent a restricted or disconnected network of vessels.
1The male Bonellia viridis has two simple nephridial tubes, each with a single internal funnel, which open externally and independently, but near the posterior attachment of the intestine. The corresponding organs in the larval Echiurus open similarly at first; their subsequent connection to the rectum is therefore secondary. It is doubtful how far the Gephyrean nephridia retain a renal function.
2 A very interesting discussion on these points will be found in Lang's 'Polycladen,' Fauna, etc. des Golfes von Neapel, xi. 1884, pp. 674-79. The occasional presence of a longitudinal canal in connection with segmental nephridia is especially noteworthy. The primitive segmental duct of Vertebrata is perhaps to be derived from it.