168 



ARION ATER. 



Diagnosis. — Arion 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 \isii:illy black or brown in tliis counti-y, but varying 

 miirvellously in its colonrinj,'; iif g'reat size, sometimes reaching 200 mill, in length 

 when extended, very bulky ami conve.K above, and terminating behind in a flattened 

 TAIL, bearing a distinct triangular GLAND; when contracted the animal assumes a 

 very characteristic heniisplicrical shape ; TUBERCLES very long and prominent, becom- 

 ing keeled and transversely wrinkled during contraction, and showing about eighteen 

 longitudinal rows at eacli side; FOOT-S(iLE obscurely tripartite, but median portion 

 not separated by a funow from the side-areas, which are usually pervaded by the 

 body colour; the FOOT-FKINGE 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 



F^ 



iiumimmmum 



■fr. 



Fig. 186. — Head o^ Arion ater, showing 

 the facial grooves. 



Fig. 187. — Foot-fringe and pedal-groove of 

 A rion ater X 2. 



p.f. pedal furrow ; fr. fringe. 



vermicular rugosities, which may coalesce into irregular and limited wavy lines, not 

 unlike the ridges of Liiim.i: ; respiratory orifice large and round, situate on the 

 ri^ht 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 liecome quadripartite on the forehead and vary greatly in shape when 

 the month is in motion ; 0iMM.\T0Pll0HES dusky, tubercled, rather swollen at the 

 apex ; lower TENTACLES also dusky and finely granulate. 



Mucus very tenacious and variable in colour; when the animal is scalded it is 

 Usually orange-coloured in red animals, in black animals usually colourless or 

 milky-white. The mucus secreted by the caudal gland is ropy and clear yellowish. 



SilBLL qnite vestigial, and generally represented liy 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 

 U-ss coherent. In the chalk-pits about Dorking Mr. Darbishire has observed that 

 all the animals contain thick and solid sljolly masses. 



Fig 

 Fig. 190, 



Fig. 189. Fig. 190. 



189.— Heart o^ylrionater X 3. mi. auricle ; 7'. ventricle. 



Fig. 188. — PaMial organs of 

 /( rion ater, as been from below X 'J 

 (after Godwin- Austen), c.r. cephalic 

 retractor ; t.r. tentacular retractors ; 

 /.;-. R(;nit,il retractor, cut short ; /,•. 

 kidficy, enclosing heart ; ?-c. rectum; 

 r,o. respiratory orifice. 



-Nerve-ring of .^.rt/drrv.r;(/fl (after I\Ioquin-Tandon). 



Intehnallv, the body caN-ity is pale and closely 

 beset with minute calcareous particles ; the buccal 

 ganglia are oval; the ilEAur is surrounded by 

 the'KiDNFV, the auricle is broadly united to the 

 ventricle, and directed towards the anterior left 

 side of the hody ; RESPiR.\T0UY ORIFICE ample, 

 allowing the complex and prominent respiratory 

 vessels to be perceptible through the opening. 



