Among the Archiannelida, in Aeolosoma and some Polychaetes, the whole central nervous system remains imbedded in the epidermis.
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.
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.
- The alimentary canal of Polychaetes is usually a straight tube running from the anterior mouth to the posterior anus.
In other Polychaetes one or more pairs of similar outgrowths are glandular.
Budding, in fact, among the Polychaetes is limited to the family Syllidae.
- " Errant" Polychaetes with well-marked prostomium possessing tentacles and palps with evident and locomotor parapodia, supported (with few exceptions) by strong spines, the aciculi; muscular pharynx usually armed with jaws; septa and nephridia regularly metameric and similar throughout body; free living and predaceous.
Still, Lake Baikal has a seal (Phoca vitulina, Phoca baikalensis of Dybowski) quite akin to the seals of Spitsbergen, marine sponges, polychaetes, a marine mollusc (ancilodoris), and some marine gammarids.
Beneath this again is a distinct peritoneum lining the coelom, which appears to be wanting as a special layer in some Polychaetes (Benham, Gilson).