They are divisible into the Haplodrili or Archiannelida, the Polychaeta containing the marine worms, the Oligochaeta or terrestrial and fresh-water annelids (see Earthworm), the Hirudinea or leeches (see Leech), and a small group of parasitic worms, the 11-Tyzostomida (q.v.).
This is the typical arrangement, which is exhibited in the majority of the Polychaeta and Oligochaeta; in these the successive chambers of the coelom are separated by the intersegmental septa, sheets of muscle fibres extending from the body wall to the gut and thus forming partitions across the body.
Thus, among the Oligochaeta there are often a series of dorsal pores, or a single head pore, present also among the Polychaeta (in Ammochares).
There are, however, further complications in some forms. Especially are these to be seen in the more modified Oligochaeta and in the much more modified Hirudinea.
Among the Oligochaeta the dorsal vessel in Dinodrilus and Megascolides is enclosed in a separate coelomic chamber which may or may not communicate with the main coelomic cavity.
These two vessels in the Oligochaeta are united in the anterior region of the body by a smaller or greater number of branches which surround the oesophagus and are, some of them at least, contractile and in that case wider than the rest.
The capillaries sometimes (in many leeches and Oligochaeta) extend into the epidermis itself.
Among many Oligochaeta the dorsal blood-vessel is partly or entirely a double tube, which is a retention of a character shown by F.
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.
- Excretory organs which are undisputed nephridia are practically universal among the Oligochaeta, Hirudinea and Archiannelida, and occur in many Polychaeta.
D, Optical section of a branch of 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.
More usually, and indeed in nearly every other case among the Oligochaeta and Hirudinea, the coelomic aperture of the nephridium consists of several cells, ciliated like the nephridium itself for a greater or less extent, forming a funnel.
2) is like that of the Oligochaeta and Hirudinea in that the coiled glandular tube has an intracellular duct which is ciliated in the same way in parts.
Though the paired arrangement of the nephridia is the prevalent one in the Chaetopoda, there are many examples, among the Oligochaeta, of species and genera in which there are several, even many, nephridia in each segment of the body, which may or may not be connected among themselves, but have in any case separate orifices on to the exterior.
In this category are included (by Goodrich and Lankester) the gonad ducts of the Oligochaeta, certain funnels without any aperture to the exterior that have been detected in Nereis, &c., funnels with wide and short ducts attached to nephridia in other Polychaeta, gonad ducts in the Capitellidae, the gonad ducts of the leeches.
Oviducts of Oligochaeta, sperm ducts of Phreoryctes, the coelomoducts occupy, like the nephridia, two segments, the funnel opening into that in front of the segment which carries the external pore.
In the case of many Oligochaeta where there is no vascular network surrounding the nephridium, this function must be the chief one of those glands, the more elaborate process of excretion taking place in the case of nephridia surrounded by a rich plexus of blood capillaries.
It has been shown that in Tubifex, and some other aquatic Oligochaeta, the genital segments are at first provided with nephridia, and that these disappear on the appearance of the generative ducts, which are coelomoducts.
In many, perhaps most, genera of the Oligochaeta family, Eudrilidae, and possibly the gonad ducts in the Hirudinea.
Nephridia sometimes of the type of those of the Oligochaeta; in other cases short, wide tubes with a large funnel serving also entirely or in part as gonad ducts.
No specialized system of spermathecae, sperm reservoirs, and copulatory apparatus, as in Oligochaeta; development generally through a larval form; reproduction by budding also occurs.
The Polychaeta contrast with the Oligochaeta by the great variety of outward form and by the frequency of specialization of different regions of the body.
As in the Oligochaeta the peristomial segment is often without setae; but this character is not by any means so constant as in the Oligochaeta.
It is a remarkable and newly-ascertained fact that in regeneration (in Potamilla) the thorax is not replaced by the growth of uninjured thoracic segments; but that the anterior segments of the abdomen take on the same characters, the setae dropping out and being replaced in accordance with the plan of the setae in the thorax of uninjured worms. Among the Oligochaeta the sexually mature worm is distinguished from the immature worm by the clitellum and by the development of genital setae.
They contrast with those of the Oligochaeta and Hirudinea by reason of their frequently close association with the gonads, the same organ sometimes serving the two functions of excretion and conveyance of the ova and spermatozoa out of the body.
On the hypothesis that such a form as Dinophilus (see Haplodrili) has preserved the characters of the primitive Chaetopod more nearly than a any existing Polychaet or Oligochaet, it is clear that the nephridia in the Oligochaeta have preserved the original features of those organs more nearly than most Polychaeta.
Thus Nereis among the latter worms, from the resemblance which its excretory system bears to that of the Oligochaeta, may be made the starting-point of a series.
Among the Capitellidae, which in several respects resemble the Oligochaeta, wide and short gonad ducts coexist in the same segments with nephridia, the latter being narrower and longer.
- As is the case with the Oligochaeta, the Polychaeta furnish examples of species which multiply asexually by budding.
As is very frequently the case with marine forms, as compared with their fresh-water and terrestrial allies, the Polychaeta differ from the Oligochaeta and Hirudinea in possessing a free living B FIG.
- As contrasted with the other subdivisions of the Chaetopoda, the Oligochaeta may be thus defined.
The Oligochaeta show a greater variety of size than any other group of the Chaetopoda.
As a rule - to which, however, there are exceptions - the clitellum consists of two cr three segments only in the small aquatic Oligochaeta, while in the terrestrial forms it is as a general rule, to which again there are exceptions, a more extensive, sometimes much more extensive, region.
In the Oligochaeta there is a closer correspondence between external metamerism and the divisions of the coelom than is apparent in some Chaetopods.
There are no renal organs with a wide intercellular lumen, such as occur in the Polychaeta, nor is there ever any permanent association between nephridia and ducts connected with the evacuation of the generative products, such as occur in Alciope, Saccocirrus, &c. In these points the Oligochaeta agree with the Hirudinea.
It has been ascertained that the nephridia of Oligochaeta are preceded in the embryo by a pair of delicate and sinuous tubes, also found in the Hirudinea and Polychaeta, which are larval excretory organs.
It is not quite certain whether these are to be regarded as the remnant of an earlier excretory system, replaced among the Oligochaeta by the subsequently developed paired structures, or whether these "head kidneys" are the first pair of nephridia precociously developed.
On the other hand, in most Oligochaeta the first segment has in the adult no nephridium, and in the case of Octochaetus the existence of a "head kidney" antedating the subsequently developed nephridia of the first and other segments has neither been seen nor proved to be absent.
The paired disposition of these organs is the prevalent one among the Oligochaeta, and occurs in all of twelve out of the thirteen families into which the group is divided.
The Oligochaeta are the only Chaetopods in which undoubted nephridia may possess a relationship with the alimentary canal.
- The Oligochaeta agree with the leeches and differ from most Polychaeta in that they are hermaphrodite.
The Oligochaeta contrast with the Polychaeta in the general presence of outgrowths of the septa in the genital segments, which are either close to, or actually involve, the gonads, and into which may also open the funnels of the gonad ducts.
These structures appear to be absolutely distinctive of the Oligochaeta, unless the sacs which contain sperm and open in common with the nephridia of Saccocirrus (see Haplodrili) are similar.
Spermathecae are generally present in the Oligochaeta and are absent only in comparatively few genera and species.
Among the aquatic Oligochaeta and many earthworms (the families Lumbricidae, Geoscolicidae and a few other genera) the spermathecae are simple structures, as has been described.
We have thus the replacement of a spermatheca, corresponding to those of the remaining families of Oligochaeta, and derived, as is believed, from the epidermis, by a structure performing the same function, but derived from the mesoblastic tissues, and with a cavity which is coelom.
They are small to moderatesized Oligochaeta, with a smaller number of segments than in the Terricolae.
- Moderate-sized to very large Oligochaeta, terrestrial in habit, with the appearance of Terricolae.
As an appendix to the Oligochaeta, and possibly referable to that group, though their systematic position cannot at present be determined with certainty, are to be placed the Bdellodrilidae (Discodrilidae auct.), which are small parasites upon crayfish.
These worms lay cocoons like the Oligochaeta and leeches, and where they depart from the structure of the Oligochaeta agree with that of leeches.