They possess (save for certain Archiannelida, most Hirudinea, and other very rare exceptions) setae or chaetae implanted in epidermal pits.
The setae are implanted metamerically in accordance with the metamerism of the body, which consists of a prostomium followed by a number of segments.
The setae ar ° invariably formed each within an epidermic cell, and they are sheathed in involutions of the epidermis.
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.
Setae always present and often very large, much varied in form and very numerous, borne by the dorsal and ventral parapodia (when present).
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.
Typically, the parapodium consists of two processes of the body on each side, each of which bears a bundle of setae; these two divisions of the "limb" are termed.
- 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.
In some genera the setae are in vertical rows, and in certain Capitellidae these rows so nearly meet that an arrangement occurs reminiscent of the continuous circle of setae in the perichaetous Oligochaeta.
The setae vary much in form and are often longer and stronger than in the Oligochaetes.
Jointed setae and very short hooks or "uncini" (see fig.
3) are among the most remarkable forms. Simple bifid setae, such as those of Oligochaetes, are also present in certain forms.
In these forms the bundles of setae are either capilliform or uncinate, and the dorsal setae of the thorax are like the ventral setae of the abdomen.
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.
The chief modifications of this form are seen in the Mitraria larva of Ammochares with only the preoral band, which is much folded and which has provisional and long setae; the a.trochous larva, where the covering of cilia is uniform and not split into bands; and the polytrochous larva where there are several bands surrounding the body.
- Tube-dwelling with body divided into thorax and abdomen marked by the setae, which are reversed in position in the neuropodium and notopodium respectively in the two regions.
The peristomium has no setae, and the setae generally are hair-like or uncinate, often forming almost complete rings.
The genital ducts are limited to one segment (the 8th in Capitella capitata), and there are genital setae on this and the next FIG.
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.
I more comp complicated in the In all the figures the nephridial pores are indicated by dots and the setae by larger forms than in the strokes.
The setae, which are always absent from the peristomial segment, are also sometimes absent from a greater number of the FIG.
- Setae of Oligochaeta.
F, g, Setae of Bohemilla comata.
H, i, j, Setae of Psammoryctes barbatus (f to jaf ter Vezhdovsky) .
When present they are either arranged in four bundles of from one to ten or even more setae, or are disposed in continuous lines completely encircling each segment of the body.
It has been shown (Bourne) that the "perichaetous" condition is probably secondary, inasmuch as in worms which are, when adult, "perichaetous" the setae develop in pairs so that the embryo passes through a stage in which it has four bundles of setae, two to each bundle, the prevalent condition in the group. Rarely there is an irregular disposition of the setae which are not paired, though the total number is eight to a segment (fig.
The varying forms of the setae are illustrated in fig.
The external segments are usually definable by the setae; and if the setae are absent, as in the anterior segments a, Penial seta of Perichaeta ceylonica.
Associated with these glands are frequently to be found bundles or pairs of long and variously modified setae which are termed penial setae,to distinguish them from other setae sometimes but not always associated with rather similarglandswhich are found anteriorly to these, and often in the immediate neighbourhood of the spermathecae; the latter are spoken of as genital setae.
The setae are either entirely capillary or there are in addition some sigmoid setae even with bifid free extremities.
There are no setae and apparently only two pairs of nephridia, of which the anterior pair open commonly by a common pore on the third segment after the head, whose segments have not been accurately enumerated.
- The leeches are more particularly to be compared with the Oligochaeta, and the following definition embraces the main features in which they agree and disagree with that group. Setae are only present in the genus Acanthobdella.
The absence of setae - save in Acanthobdella, where five of the anterior segments possess each four pairs of setae with reserve setae placed close behind them (fig.
The absence of setae and the great secondary annulation render the mapping of the segments a subject of some difficulty.
- Acanthobdella, from the ventral surface, showing the five sets of setae (S 1 to S5) and the replacing setae (Sr) behind them.
Paired setae of Oligochaetous pattern present in anterior segments.
The free edges of the mantle often bear chitinous bristles or setae which project beyond the shell.
In it the hairs are confined to the dorsal middle line and the creeping setae are hooked, of a finer structure than in Chaetosoma, and situated so far forward that the vagina opens amongst them.
The body-wall is highly muscular and, except in a few probably specialized cases, possesses chitinous spines, the setae, which are secreted by the ectoderm and are embedded in pits of the skin.
The " postabdomen," marked off by the two postabdominal setae, usually has teeth or spines, and ends in two denticulate or ciliate claws, or it may be rudimentary, as in Polyphemus.
The former, with the feet for the most part concealed by the carapace, is subdivided into two tribes, the Ctenopoda, or " comb-feet," in which the six pairs of similar feet, all branchial and nonprehensile, are furnished with setae arranged like the teeth of a comb, and the Anomopoda, or " variety-feet," in which the front feet differ from the rest by being more or less prehensile, without branchial laminae.
No other member of the group is known to have any trace of setae or parapodia at any stage of development.
A, ap, c, c.o, c.t, d.v, e, ep, g Whatever may be the conclusion as to the position of Dinophilus and Histriodrilus, it seems only reasonable to suppose that Polygordius and Protodrilus, so far from representing a stage in the phylogeny of the Annelida before setae were developed, have lost the setae, which are already in a reduced state in Saccocirrus.
As in Arthropoda, the hairs or setae on the surface of the body are important organs of sense and are variously modified for special sensory functions.
When feathered or provided with secondary barbs the setae will respond to movements or vibrations in the surrounding water, and have been supposed to have an auditory function.
In the majority of the Decapoda there is a saccular invagination of the integument in the basal segment of the antennular peduncle having on its inner surface " auditory " setae of the type just described.
Another modification of sensory setae is supposed to be associated with the sense of smell.
The dorsal surface is smooth; ventrally there are five pairs of parapodia, armed with supporting and hooked setae, by means of which the worm adheres to its host.