168 ARION ATER. 
Diagnosis.—-rion ater may be distinguished when adult by its large 
size, long and coarse tubercules, and usually uniform colouration, as well as 
by the very contracted and hemispherical shape it assumes when at rest 
and particularly when alarmed or irritated, a position often accompanied 
by a very peculiar swaying or elephantine motion of the body. 
INTERNALLY, this species is perhaps best distinguished from its con- 
geners by the greatly enlarged and protuberant base of the oviduct. 
Description.—ANIMAL usually black or brown in this country, but varying 
marvellously in its colonring; of great size, sometimes reaching 200 mill. in length 
when extended, very bulky and convex above, and terminating behind in a flattened 
TAIL, bearing a distinct triangular GLAND; when contracted the animal assumes a 
very characteristic hemispherical shape ; TUBERCLES very long and prominent, becom- 
ing keeled and transversely wrinkled during contraction, and showing about eighteen 
longitudinal rows at each side; FOOT-SOLE obscurely tripartite, but median portion 
not separated by a furrow from the side-areas, which are usually pervaded by the 
body colour; the FOOT-FRINGE is broad, widest at the tail, spread out when the 
animal is at rest, and presenting a series of transverse parallel lineoles, of which 
every alternate one is darker; in strongly coloured individuals these lines are con- 
tinued across the side-areas of the sole ; SHIELD ovoid, narrow in front, with small 
Fic. 187.—Foot-fringe and pedal-groove of 
Fic. 186.—Head of Avion ater, showing Arion ater X 2. 
the facial grooves. pS. pedal furrow ;_/r. fringe. 
vermicular rugosities, which may coalesce into irregular and limited wavy lines, not 
unlike the ridges of Limaxv ; RESPIRATORY ORIFICE large and round, situate on the 
right side of the body, near the anterior third of the length of the shield, its lower 
margin cut by the anal channel; HEAD usually darker than the rest of the body, 
with four dorsal furrows, the outer ones terminating at the ommatophores, but the 
median pair become quadripartite on the forehead and vary greatly in shape when 
the mouth is in motion; OMMATOPILORES dusky, tubereled, rather swollen at the 
apex ; lower TENTACLES also dusky and finely granulate. 
Mucus very tenacious and variable in colour; when the animal is sealded it is 
usually orange-coloured in red animals, in black animals usually colourless or 
milky-white. The mueus secreted by the caudal gland is ropy and clear yellowish. 
SHELL quite vestigial, and generally represented by a soft and pulpy calcareous 
deposit placed beneath the hinder part of the shield ; it is, however, sometimes more 
concentrated and becomes hardened into a granular form, with the particles more or 
less coherent. In the chalk-pits about Dorking Mr. Darbishire has observed that 
all the animals contain thick and solid shelly masses. 
Fic. 189. Fic. 190. 
Fic. 189.—Heart of . 17ton ater X 3. au. auricle; v. ventricle. 
Fic. 190.—Nerve-ring of A. aterv.rufa(after Moquin-Tandon). 
INTERNALLY, the body cavity is pale and closely 
beset with minute calcareous particles ; the buccal 
Fic. 18h lead areans (Or ganglia are oval ; the HEART is surrounded by 
Arion ater, as seen from below x 2. the KIDNEY, the auricle is broadly united to the 
(after Godwin-Austen). c.» cephalic ventricle, and directed towards the anterior left 
retractor; ‘oe Paes N es aag side of the BODY; RESPIRATORY ORIFICE ample, 
Coe een eee enacts. allowing the complex and prominent respiratory 
kidney, enclosing heart; 7c. rectum; ‘ : 
7.0. respiratory orifice. vessels to be perceptible through the opening. 
