Amongst the Nereidiformia the remains are fewer, but the acicula and the hard jaws are preserved in certain rocks, and can be referred to existing families. Eunicites avitus[[363]] is represented by a double series of acicula, indicating the parapodia of the two sides; and by remains of both upper and lower jaws (Fig. 159). Four different species of the worm have been described from the lithographic slate of Bavaria, of Jurassic age; and several upper jaws of other Eunicids have been discovered in the Palaeozoic beds of Canada and Scotland, and have received the names Lumbriconereites, Oenonites, and Arabellites, in reference to their nearest allies amongst living genera.
There are, however, numerous remains, in the forms of tracks or casts, in the earlier rocks, which have been referred to the Polychaeta. The names Crossopodia, Myrianites, Nereites, Phyllodocites, have been given to some of these traces, though they are open to numerous other interpretations. Some of the "tracks" are similar to those made by living Crustacea in walking over wet sand; others appear to be the casts of some animals. Tubular burrows in rocks or fossils, some straight, others U-shaped, have received such names as Arenicolites, Scolithus, Histioderma; whilst under the name Lumbricaria certain cylindrical, coiled structures, resembling worm "castings," are met with in this same lithographic stone of Solenhofen. Many of the tubes referred to Polychaetes by the earlier palaeontologists have been transferred to other groups; thus Cornulites is now believed to be a Pteropod shell.
This very meagre geological record is quite insufficient to form any basis for a phylogeny of the group. And this poor supply of remains is not surprising, when we consider the soft nature of the tissues, the absence, in the majority of families, of skeleton and of other parts which could have been fossilised; yet we might have expected a greater abundance of fossilised jaws than is represented at present. But it must be borne in mind that the conditions of life of these soft-bodied animals are not conducive to their leaving abundant fossilised remains.
CHAPTER XII
CHARACTERS OF THE SUB-ORDERS OF POLYCHAETES—CHARACTERS OF THE FAMILIES—DESCRIPTION OF BRITISH GENERA AND SPECIES—THE MYZOSTOMARIA.
Systematic.—The Order Polychaeta may be divided into two branches, in one of which, the Phanerocephala, the prostomium retains its ancestral condition as a lobe overhanging the mouth, and frequently carries, in addition to paired eyes, certain sensory processes of a simple structure, the tentacles and palps; the body-segments are more or less alike, and (except in some Spioniformia, some of the Terebelliformia, and the Capitelliformia) do not present two sharply marked regions, owing to the differential arrangement or character of the chaetae. In the second branch, the Cryptocephala, the peristomium grows forwards during development, so as to compress or even hide the prostomium, which thus becomes a very insignificant organ. The tentacles are reduced, but the palps become greatly developed and take on sundry new functions. The body in this group, by the character and arrangement of the chaetae, is distinguishable into a thorax and abdomen, presenting certain internal differences.
These two branches may be supposed to have arisen from a common ancestor having a general resemblance to a nereidiform worm, such as Syllis, possessing palps and tentacles on the prostomium, definite parapodia and cirri on the body, and internally, a well-marked and regular repetition of organs.
The branch Phanerocephala contains the following five sub-Orders, though it is possible that the Capitelliformia deserves a more important position in the system:—
Sub-Order 1.—The Nereidiformia have well-developed tentacles and palps; the peristomium almost invariably possesses special cirri; the parapodia are well-marked locomotor organs, supported by acicula, and carry dorsal and ventral cirri. The chaetae are usually jointed, though unjointed ones may coexist with these; uncini are never present. An eversible buccal region leads into a muscular pharynx, which in the majority is armed with chitinous jaws; the septa and nephridia are regularly repeated throughout the body. The worms lead a predaceous life, and are mostly carnivorous; a few form tubes.
Sub-Order 2.—The Spioniformia possess neither tentacles nor palps; the peristomium usually carries a pair of long tentacular cirri, and extends forwards at the sides of the prostomium. The parapodia project only to a slight degree; the dorsal cirri may attain a considerable size, and act as gills throughout the greater part of the body. The chaetae are unjointed; uncini are only present in the aberrant Chaetopterus.[[364]] The body may present two regions more or less distinctly marked externally, but without corresponding internal differences. The buccal region may be eversible, but there are no jaws. Septa and nephridia are regularly developed. The worms are burrowers, or tubicolous.