respectively notopodium and neuropodium. The notopodium may be rudimentary or absent and the entire parapodium reduced to the merest ridge or even completely unrepresented. Naturally, it is among the free living forms that the parapodium is best developed, and least developed among the tubicolous Polychaeta. To each division of the parapodium belongs typically a long tentacle, the cirrus, which may be defective upon one or other of the notopodium or neuropodium, and may be developed into an arborescent gill or into a flat scale-like process, the elytron (in Polynoe, &c.). There are other gills developed in addition to those which represent the cirri.
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| Fig. 3.—a, Bristle of Pionosyllis Malmgreni; b, Hook of Terebella. | |
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. Syllis). 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.
Among the burrowing and tubicolous forms it is not uncommon for the body to be distinguishable into two or more regions; a “thorax,” for example, is sharply marked off from an “abdomen” in the Sabellids. 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. 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 alteration in form does not only affect structures used in generation; but the form of the parapodia, &c., alter. There are even dimorphic forms among the Syllids where the sexes are, as in many Polychaets, separate.
Nephridia.—The nephridia of the Polychaeta have been generally dealt with above in considering the nephridial system of the Chaetopoda as a whole. 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 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. In this worm the paired nephridia exist in most of the segments of the body, and their form (see fig. 2) is much like that of the nephridia in the Enchytraeidae. The funnel, which is not large, appears to open, as a rule at least, into the segment in front of that which bears the external orifice. Quite independent of these are certain large dorsally situate funnel-like folds of the coelomic epithelium, ciliated, but of which no duct has been discovered leading to the exterior. It is possible that we have here gonad ducts distinct from nephridia which at the time of sexual maturity do open on to the exterior.
In Polynoe the nephridia are short tubes with a slightly folded funnel whose lumen is intercellular, and this intercellular lumen is characteristic of the Polychaetes as contrasted with leeches and Oligochaetes. Among the Terebelloidea there is a remarkable differentiation of the nephridia into two series. One set lies in front of the diaphragm, which is the most anterior and complete septum, the rest having disappeared or being much less developed. The anterior nephridia, of which there are one to three pairs, contrast with the posterior series by their small funnels and large size, the posterior nephridia having a large funnel followed by a short tube. In Chaetozone setosa the anterior nephridia occupy five segments. There is usually a gap between the two series, several segments being without nephridia. It seems that the posterior nephridia are mainly gonad ducts, and the gonads are developed in close association with the funnels. The same arrangement is found in some other Polychaetes; for instance, in Sabellaria there is a single pair of large anterior nephridia, which open by a common pore, followed after an interval by large-funnelled and short nephridia. This differentiation is not, however, peculiar to the Polychaetes; for in several Oligochaetes the anterior nephridia are of large size, and opening as they do into the buccal cavity clearly play a different function to those which follow. In Thamnodrilus, as has been pointed out, there are two series of nephridia which resemble those of the Terebelloidea in the different sizes of their funnels. In Lanice conchilega the posterior series of nephridia are connected by a thick longitudinal duct, which seems to be seen in its most reduced form in Owenia, where a duct on each side runs in the epidermis, being in parts a groove, and receives one short tubular nephridium only and occupies only one segment. This connexion of successive nephridia (in Lanice) has its counterpart in Allolobophora, Lybiodrilus, and apparently in the Lumbriculids Teleuscolex and Styloscolex, among the Oligochaeta. 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. 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.
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Fig. 4.—Dasychone infracta, Kr. (After Malmgren.) |
Alimentary Canal.—The alimentary canal of Polychaetes is usually a straight tube running from the anterior mouth to the posterior anus. But in some forms, e.g. Sternaspis, the gut is coiled. In others, again, e.g. Cobangia, the anus is anterior and ventral. A gizzard is present in a few forms. The buccal cavity is sometimes armed with jaws. The oesophagus is provided often with caeca which in Syllids and Hesionidae have been found to contain air, and possibly therefore perform the function of the fish’s air-bladder. In other Polychaetes one or more pairs of similar outgrowths are glandular. The intestine is provided with numerous branched caeca in Aphrodite.
Reproduction.—As is the case with the Oligochaeta, the Polychaeta furnish examples of species which multiply asexually by budding. There is a further resemblance between the two orders of Chaetopoda in that this budding is not a general phenomenon, but confined to a few forms only. Budding, in fact, among the Polychaetes is limited to the family Syllidae. In the Oligochaetes it is only the families Aeolosomatidae and Naididae that show the same phenomenon. It has been mentioned that in the Nereids a sexual form occurs which differs structurally from the asexual worms, and was originally placed in a separate genus, Heteronereis; hence the name “Heteronereid” for the sexual worm. In Syllis there is also a “Heterosyllid” form in which the gonads are limited to a posterior region of the body which is further marked off from the anterior non-sexual segments by the oak-like setae. In some Syllids this posterior region separates off from the rest, producing a new head; thus a process of fission occurs which has been termed schizogamy. A similar life history distinguishes certain Sabellid worms, e.g. Filigrana. Among the Syllids this simple state of affairs is further complicated. In Autolytus there is, to begin with, a conversion of the posterior half of the body to form a sexual zooid. But before this separates off a number of other zooids are formed from a zone of budding which appears between the two first-formed individuals. Ultimately, a chain of sexual zooids is thus formed. A given stock only produces zooids of one sex. In Myrianida there is a further development of this process. The conversion of the posterior end of the simple individual into a sexual region is dispensed with; but from a preanal budding segment a series of sexual buds are produced. The well-known Syllid, discovered during the voyage of the “Challenger,” shows a modification of this form of budding. Here, however, the buds are lateral, though produced from a budding
