The resin contains, in addition to the beautifully preserved plant-structures, numerous remains of insects, spiders, annelids, crustaceans and other small organisms which became enveloped while the exudation was fluid.
In external characters the Hirudinea are unmistakable and not to be confused with other Annelids, except perhaps with the Bdellodrilidae, which resemble them in certain particulars.
Pharynx, and he sums up their relationship to the Annelids by thestatement that to a certain extent the Nemertines represent Turbellaria which in the course of time have copied certain features of an Annelid character.
Its integument is marked by a large number of transverse grooves simulating the segmentation of Annelids, and near the anterior extremity close to the mouth are two pairs of recurved chitinous hooks.
One can imagine the interest and astonishment with which the great Greek would have been filled had some unduly precocious disciple shown to him the red-blood-system of the marine terrestrial Annelids; the red blood of Planorbis, of Apus cancriformis, and of the Mediterranean razor shell, Solen legumen.
Annelids are segmented worms, and differ from the Arthropoda, which they closely resemble in many respects, by the possession of a portion of the coelom traversed by the alimentary canal.
The egg is holoblastic, but the segmentation is very unequal, recalling that of marine annelids and of molluscs.
Un like the molluscs and annelids, however, the cloacal invagination lies outside this region, and the foot is formed by an elongation of the end of the body between the two apertures.
The shell-bearing forms belonging to this group which were known to Linnaeus were placed by him (in 1748) in the third order of his class Vermes under the name " Testacea," whilst the Echinoderms, Hydroids and Annelids, with the naked Mollusca, formed his second order termed " Zoophyta."
It has certain histological resemblances with the Nematoda and certain primitive Annelids, but little stress must be laid on these.
Annelids (Spirorbis, Serpulites, &c.) are common fossils on certain horizons.
There appears to be little either in the development or in the structure of the Haplodrili to warrant the view held by Hatschek and Fraipont that Polygordius and Protodrilus are exceedingly primitive forms, ancestral to the whole group of seta-bearing Annelids (Oligochaeta, Polychaeta, Hirudinea and Echiuroidea).
Traces of annelids have been detected in some of the quartzites, and some of the less changed parts of the limestones may be searched for fossils.
Traces of annelids and probably other organisms have been found in the bands of shale occurring in the south-west of the shire of Ross and Cromarty, in the isle of Raasay, and at Cailleach Head, and are the oldest relics of animal life yet found in Great Britain.
Annelids left their traces in burrows and casts on the sea-floor (Arenicolites, Cruziana, Scolithus, &c.).
In the primitive Phyllopoda the ventral chain retains the ladder-like arrangement found in some Annelids and lower worms, the two halves being widely separated and the pairs of ganglia connected together across the middle line by double transverse commissures.
It is an ancestral larval form, corresponding perhaps to the stages immediately succeeding the trochophore in the development of Annelids, but with some of the later-acquired Crustacean characters superposed upon it.
At first supposed to be tubicolous Annelids, Dentalium and its allies were afterwards placed among the Gastropoda, to which recent authorities consider them to be closely related.
Aldrovandi in De animalibus insectis (1602) almost contemporaneously distinguished between "terrestrial insects," including woodlice, earthworms and slugs, and "aquatic insects," comprising annelids and starfishes.