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.).
Where locomotive appendages (the parapodia of the Polychaeta) exist, they are never jointed, as always in the Arthropoda; nor are they modified anteriorly to form jaws, as in that group.
The setae are organs of locomotion, though their large size and occasionally jagged edges in some of the Polychaeta suggest an aggressive function.
They are disposed in two groups on either side, corresponding in the Polychaeta to the parapodia; the two bundles are commonly reduced among the earthworms to two pairs of setae or even to a single seta.
On the other hand, in certain Polychaeta the bundles of setae are so extensive that they nearly form a complete circle surrounding the body; and in the Oligochaet genus Perichaeta (=Pheretima), and some allies, there is actually a complete circle of setae in each segment broken only by minute gaps, one dorsal, the other ventral.
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).
In the Polychaeta, which are to be regarded as structurally simpler forms than the two groups just referred to, there is but little subdivision of the coelom of the segments, indeed a tendency in the reverse direction, owing to the suppression of septa.
With a few exceptions among the Polychaeta the vascular system is always present among the Chaetopoda, and always consists of a system of vessels with definite walls, which rarely communicate with the coelom.
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.
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.
Finally, there are certain Polychaeta, e.g.
Among the Polychaeta the nephridium of Nereis (see fig.
The Polychaeta, however, present us with another form of nephridium seen, for example, in Arenicola, where a large funnel leads into a short and wide excretory tube whose lumen is intercellular.
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.
This example is similar to cases among the Polychaeta where a true nephridium is provided with a large funnel, a coelomostome, according to the nomenclature of Lankester.
Polychaeta.-ThiS group may be thus defined and the definition contrasted and compared with those of the other divisions of the Chaetopoda.
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.
To each division of the parapodium Polychaeta Setae.
- The setae of the Polychaeta are disposed in two bundles in many genera, but in only one bundle in such forms as have no notopodium (e.g.
Among the Polychaeta the sexual worm is often more marked from the asexual form, so much so that these latter have been placed in different species or even genera.
The nephridia of the Polychaeta have been generally dealt with above in considering the nephridial system of the Chaetopoda as a whole.
It is noteworthy that in this family only among the Polychaeta, the nephridia are not restricted to a single pair in each segment; so that the older view that the gonad ducts are metamorphosed nephridia is not at variance with the anatomical facts which have been just stated.
- 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.
The older arrangement of the Polychaeta into Errantia or free living and Tubicola or tube-dwelling forms will hardly fit the much increased knowledge of the group. W.
Benham, "Polychaeta" in Cambridge Natural History; E.
Setae very rarely absent (genus Achaeta) and as a rule not so large or so numerous in e (.r, each segment as in the is Polychaeta, and different in shape.
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.
- 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.
Saccocirrus, which also lives in sand, and more closely resembles the Polychaeta, has throughout the greater length of its body on each segment a pair of small uniramous parapodia bearing a bunch of simple setae.
There appears to be little either in the development or in the structure of the Haplodrili to warrant the view held by Hatschek and Fraipont that Polygordius and Protodrilus are exceedingly primitive forms, ancestral to the whole group of seta-bearing Annelids (Oligochaeta, Polychaeta, Hirudinea and Echiuroidea).
The structure and development of the Myzostomida seem to show that they are nearly related to Polychaeta (see Chaetopoda), though highly modified in relation to their parasitic mode of life.