Page:EB1911 - Volume 05.djvu/827

 developed only in the anterior region of the body where it emerges from the peri-intestinal blood sinus. On the other hand, additional longitudinal trunks are sometimes developed, the chief one of which is a supra-intestinal vessel lying below the dorsal vessel and closely adherent to the walls of the oesophagus in which region it appears. The capillaries sometimes (in many leeches and Oligochaeta) extend into the epidermis itself. Usually they do not extend outwards of the muscular layers of the body wall. The main trunks of the vascular system often possess valves at the origin of branches which regulate the direction of the blood flow. Among many Oligochaeta the dorsal blood-vessel is partly or entirely a double tube, which is a retention of a character shown by F. Vezhdovský to exist in the embryo of certain forms. The blood in the Chaetopoda consists of a plasma in which float a few corpuscles. The plasma is coloured red by haemoglobin: it is sometimes (in Sabella and a few other Polychaeta) green, which tint is due to another respiratory pigment. The plasma may be pink (Magelona) or yellow (Aphrodite) in which cases the colour is owing to another pigment. In Aeolosoma it is usually colourless. The vascular system is in the majority of Chaetopods a closed system. It has been asserted (and denied) that the cellular rod which is known as the “Heart-body” (Herzkorper), and is to be found in the dorsal vessel of many Oligochaeta and Polychaeta, is formed of cells which are continuous with the chloragogen cells, thus implying the existence of apertures of communication with the coelom. The statement has been often made and denied, but it now seems to have been placed on a firm basis (E. S. Goodrich), that among the Hirudinea the coelom, which is largely broken up into narrow tubes, may be confluent with the tubes of the vascular system. This state of affairs has no antecedent improbability about it, since in the Vertebrata the coelom is unquestionably confluent with the haemal system through the lymphatic vessels. Finally, there are certain Polychaeta, e g. the Capitellidae, in which the vascular system has vanished altogether, leaving a coelom containing haemoglobin-impregnated corpuscles. It has been suggested (E. Ray Lankester) that this condition has been arrived at through some such intermediate stage as that offered by Polychaet Magelona. In this worm the ventral blood-vessel is so swollen as to occupy nearly the whole of the available coelom. Carry the process but a little farther and the coelom disappears and its place is taken by a blood space or haemocoel. It has been held that the condition shown in certain leeches tend to prove that the coelom and haemocoel are primitively one series of spaces which have been gradually differentiated. The facts of development, however, prove their distinctness, though those same facts do not speak clearly as to the true nature of the blood system. One view of the origin of the latter (largely based upon observations upon the development of Polygordius) sees in the blood system a persistent blastocoel. F. Vezhdovský has lately seen reasons for regarding the blood system as originating entirely from the hypoblast by the secretion of fluid, the blood, from particular intestinal cells and the consequent formation of spaces through pressure, which become lined with these cells.

Nephridia and Coelomoducts.—The name “Nephridium” was originally given by Sir E. Ray Lankester to the members of a series of tubes, proved in some cases to be excretory in nature, which exist typically to the number of a single pair in most of the segments of the Chaetopod body, and open each by a ciliated orifice into the coelom on the one hand, and by a pore on to the exterior of the body on the other. In its earlier conception, this view embraced as homologous organs (so far as the present group is concerned) not only the nephridia of Oligochaeta and Hirudinea, which are obviously closely similar, but the wide tubes with an intercellular lumen and large funnels of certain Polychaeta, and (though with less assurance) the gonad ducts in Oligochaeta and Hirudinea. The function of nitrogenous excretion was not therefore a necessary part of the view—though it may be pointed out that there are grounds for believing that the gonad ducts are to some extent also organs of excretion (see below). Later, the investigations of E. Meyer and E. S. Goodrich, endorsed by Lankester, led to the opinion that under the general morphological conception of “nephridium” were included two distinct sets of organs, viz. nephridia and coelomoducts. The former (represented by, e.g. the “segmental organs” of Lumbricus) have been asserted to be “ultimately, though not always, actually traceable to the ectoderm”; the latter (represented by, e.g. the oviduct of Lumbricus) are parts of the coelomic wall itself, which have grown out to the exterior. The nephridia, in fact, on this view, are ectodermic ingrowths, the coelomoducts coelomic outgrowths. The cavity of the former has nothing to do with coelom. The cavity of the latter is coelom.

The embryological facts upon which this view has been based, however, have been differently interpreted. According to C. O. Whitman the entire nephridial system (in the leech Clepsine) is formed by the differentiation of a continuous epiblastic band on each side. The exact opposite is maintained by R. S. Bergh (for Lumbricus and Criodrilus), whose figures show a derivation of the entire nephridium from mesoblast, and an absence of any connexion between successive nephridia by any continuous band, epiblastic or mesoblastic. A midway position is taken up by Wilson, who asserts the mesoblastic formation of the funnel, but also asserts the presence of a continuous band of epiblast from which certainly the terminal vesicle of the nephridium, and doubtfully the glandular part of the tube is derived. Vezhdovský’s figures of Rhynchelmis agree with those of Bergh in showing the backward growth of the nephridium from the funnel cell. There are thus substantial reasons for believing that the nephridium grows backwards from a funnel as does the coelomoduct. It is therefore by no means certain that so profound a difference embryologically can be asserted to exist between the excretory nephridia and the ducts leading from the coelom to the exterior, which are usually associated with the extrusion of the genital products among the Chaetopoda.

There are, however, anatomical and histological differences to be seen at any rate at the extremes between the undoubted nephridia of Goodrich, Meyer and Lankester, and the coelomoducts of the same authors.

I. Nephridia.—Excretory organs which are undisputed nephridia are practically universal among the Oligochaeta, Hirudinea and Archiannelida, and occur in many Polychaeta. Their total absence has been asserted definitely only in Paranais littoralis. Usually these organs are present to the number of a single pair per somite, and are commonly present in the majority of the segments of the body, failing often among the Oligochaeta in a varying number of the anterior segments. They are considerably reduced in number in certain Polychaeta. Essentially, a nephridium is a tube, generally very long and much folded upon itself, composed of a string of cells placed end to end in which the continuous lumen is excavated. Such cells are termed “drain pipe” cells. Frequently the lumen is branched and may form a complicated anastomosing network in these cells. Externally, the nephridium opens by a straight part of the tube, which is often very wide, and here the intracellular lumen becomes intercellular. Rarely the nephridium does not communicate with the coelom; in such cases the nephridium ends in a single cell, like the “flame cell” of a Platyhelminth worm, in which there is a lumen blocked at the coelomic end by a tuft of fine cilia projecting into the lumen. This is so with Aeolosoma (Vezhdovský). The condition is interesting as a persistence of the conditions obtaining in the provisional nephridia of e.g. Rhynchelmis, which afterwards become by an enlargement and opening up of the funnel the permanent nephridia of the adult worm. In some Polychaets (e.g. Glycera, see fig. 2) there are many of these flame cells to a single nephridium which are specialized in form, and have been termed “solenocytes” (Goodrich). They are repeated in Polygordius, and are exactly