164 GENUS ARION. 
needing at times to be supplemented by a study of the external features 
of the animal. Eyen Dr. Simroth, whose profound knowledge of the 
organization of the Avrions is incontestable, feels compelled to remark 
“that the species of the genus Arion are so difficult to distinguish anato- 
inically, that they can only be determined with certainty by the aid of 
their colouring.” It is, therefore, to be regretted that, in view of this 
almost too closely intimate relationship, so many so-called new species 
have been described, and that painstaking naturalists of our own country 
have been found to emulate in this respect the more objectionable methods 
of the few continental extremists. 
Generic Characteristics.—ExTerNALLy, the features of the Arions 
when adult are a rounded porsuM and a somewhat corpulent BoDy ; rugose 
SKIN: an uncarinated and blunt Tar ; Foor with distinct pedal-groove 
meeting over a caudal mucus pore; MANTLE or shield granulate, rounded 
at each end, and placed at the anterior end of the body, with the RESPIR A- 
TORY APERTURE near its right anterior end, and the GENITAL ORIFICE beneath. 
According to Simroth, they originally possessed an ancestral lateral band 
at each side, coincident in position with the longitudinal lateral blood 
sinuses, but in the more advanced forms this peculiarity becomes obscured. 
The SHELL is represented by a soft and pulpy calcareous secretion, which 
solidifies upon exposure to the air, and also to some extent by age ; it is 
placed beneath the hinder part of the mantle, and according to Lankester, 
is within a permanently-closed sac, and represents the primitive shell. 
INTERNALLY, the group is characterized by the crowding of the main 
mass of the genitalia into the anterior half of the body, and the absence 
of the penis, the intromittent functions of which organ have been usurped 
by the vagina. It is also remarkable for the opaque-white colour of the 
walls of the arterial vessels, especially those investing the digestive gland 
and the alimentary canal, their complex ramifications showing out beauti- 
fully against the darker background. ‘This whiteness is due to a dense 
deposit of fat and lime within the cells of the arterial sheath, a deposit 
regarded by Semper as a temporary storehouse of lime for subsequent use 
in the body. 
‘he NERVOUS SYSTEM is remarkable for the distribution of the dorsal 
nerve, which becomes bifid apparently in correlation with the separation 
of the TENTACULAR RETRACTORS with which they are in association. 
‘he ALIMENTARY CANAL is more or less spirally triodromous, the coil beg 
held in position anteriorly by the aorra, as is usual, and the stoMacH 
Tract being the most posteriorly extended ; the saw is odontognathous’ 
or ribbed; and the teeth of the RapuLA cuspidate, with quadrate base 
of attachment. 
he MUSCULAR SYSTEM is quite different from that displayed by Limaaz 
and by the Helicidw, in which the tentacular and pharyngeal muscles 
unite posteriorly into a single band or possess a common base of attachment, 
whereas in Arion the pharyngeal as well as each tentacular muscle have 
their separate and widely-distant places of attachment to the dorsal skin, 
and constitute the section 'richoriza.2 This separation of the tentacular 
retractors is probably due to mechanical causes, the oblique strain tending 
to pull apart the muscles, and as the soft degenerate shell no longer 
affords a firm attachment, the retractors have become fixed to the tough 
dorsal integument or to the lung floor. 
1 Monog, i., p. 254, f. 509, 2 Monog. i., p. 344, f. 638, 
