Description (from Franklin, 1955)
The cuticle, except on the head, is annulated, the annules being about 1-1.3 um apart in the mid-body region. The lateral field bears four equally-spaced longitudinal incisures which, shortly behind the oesophageal region and at the beginning of the tail, are reduced in number first to three, then to one which finally disappears. No deirid could be seen and phasmids were observed only with difficulty. The body tapers gradually towards the head which is distinctly offset, the sides being convex but slightly less so than in A. ritzema-bosi. There appear to be no superficial longitudinal marks on the head indicating the lip divisions, such as have sometimes been represented in drawings of the nematode. A face view of the head seen by incident light shows six lips, but papillae are not visible and the amphidial openings are not at all clear. The excretory pore is on the ventral surface close behind the esophageal bulb, but no hemizonid could be seen. The anus is distinct and behind it the tail tapers fairly rapidly to end in a mucro which is usually a simple spike but may be somewhat truncate. When the nematode is killed by gentle heat the tail curves ventrally in a characteristic way approaching the form typical of the male of A. fragariae. The vulva is situated at two-thirds of the total length from the head: it is a simple transverse slit.
The stylet is 11-13 um long (average of 8 = 11.75 um) and is distinctly thickened at the base, the thickenings being smaller than the knobs of A. ritzema-bosi and A. blastophthorus and not so sharply offset. A guide ring surrounds the tip of the stylet, appearing star-shaped when seen from the front and as two short rods when seen from the side. The first region of the oesophagus is narrow with a well marked lumen and is followed by a prominent median bulb occupying about two-thirds of the width of the body at the same point. The bulb is longer than broad and slightly wider at the posterior than at the anterior end. In the center there is a conspicuous valve with crescentic thickenings. A duct from the oesophageal gland opens into the lumen just in front of the valve and two other ducts presumably enter behind it in a ventrolateral plane, since occasionally it is possible to see a break in the wall of the lumen at this point. The elongated oesophageal gland is in length about four times the greatest body width and lies dorsally along the intestine, closely adpressed but separate from it. The intestine begins immediately behind the esophageal bulb as a tube about one quarter to one-third the width of the bulb and, gradually widening, runs straight to the narrow rectum which has a length equal to about twice the anal body-width. The intestine walls are usually well stocked with oil globules. The nerve ring surrounds the intestine and oesophageal gland close behind the median bulb.
The ovary is straight and consists of a single row of about a dozen oocytes, the tip usually lying a short distance behind the end of the oesophageal gland but sometimes overlapping it. The oviduct is short and in a few specimens a single egg was seen in the uterus. The egg is about 4 times as long as it is broad. There is a small post-vulval sac, usually about 2 body widths long, but neither here nor in the uterus could sperms be seen.
No males have been found.
A. parietinus is a common inhabitant of Xanthoria parietina and has also been found in the green foliose species Cladonia fimbriata var. simplex. Within the lichens it must be able to withstand considerable desiccation. On the assumption that it feeds on the fungal constituent of the lichen attempts were made to culture it with a fungus on agar plates. On one occasion females, eggs and larvae were found after a period of about three months, but there was no sign of males. It seems probable that this species is mycophagous.