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 {304}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.