As in the Oligochaeta the peristomial segment is often without setae, but this character is not by any means so constant as in the Oligochaeta.
It is, however, always possible to recognize a head, which consists at least of the peristomial segment with a forward projection of the same, the prostomium.
The setae, which are always absent from the peristomial segment, are also sometimes absent from a greater number of the anterior segments of the body, and have completely disappeared in Achaeta cameranoi.
With reference to the head and its appendages it has already been stated that the head is primarily formed of the prae-oral lobe and of the peristomial region.
The embryonic peristomial region may, on the other hand, be regarded as in a certain sense the first segment.
In the present species the prostomium is proportionately larger, less covered by the peristomial flap which does not extend over the base of the median tentacle.
Eyes large; in type orange colored; the two on each side contiguous or nearly so; posterior ones nearer together, each beneath edge of the quadrate peristomial flap, while the anterior ones are in line with base of median tentacle.
The mouth lies slightly above the center of the body and marks the posterior limit of the ventral peristomial groove, which curves slightly from the anterior extremity.
A peristomial depression leading to the mouth is absent or very slightly indicated.
The mouth is large and placed at the apex of the peristomial triangle and opens into a comparatively small oesophagus.
The mouth is placed centrally or near the left margin, and from it the right edge of the peristome forms a curved line to the left, which bends forward, thus making the greater part of the left edge the peristomial area.
According to Stein, the two edges can approach each other, thus opening and closing the peristomial area.
The peristomial depression is Family Ophryoscolecidæ deep and funnel-like; cuticle thick, with posterior spine-like processes.
The mouth, as in the preceding species, is at the end of a long peristomialgroove extending from the anterior end to the middle of the body.
The bodies are variable--peristomial border widely dilated, tapering and attenuate posteriorly.
Peristomial depression short; Family Stentoridæ limited to the anterior end; its plane at right angles to the long axis of body; surface of peristome striated and ciliated; no undulating membranes 2.
Mouth anterior or central; Family Chiliferidæ pharynx short or absent; peristomial depression faint or absent b.
The mouth terminates a long Family Plagiotomidæ peristomial furrow having an adoral zone along the entire left edge b.
Length 60µ from disk to extremity of the peristomial disk.
The latter is surrounded by a complete circle of the adoral zone, the oral end of which passes into a peristomial depression which extends deep into the middle of the body.
Wreath of distinct membranelles--or of cilia fused at the base--enclosing a peristomial area and leading into the mouth.
Cilia nearly uniform, not associated with cirrhi or membranelles, nor forming a peristomial wreath.
Peristomial wreath projecting when expanded above a circular contractile collar-like rim.
Peristomial wreath, not protrusible, surrounded by a very high usually spiral collar.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "peristomial" in a variety of sentences. We hope that you will now be able to make sentences using this word.
