1G4 GENUS ARION. 



ueediug at times to be siipiilemeiited by. a study of the external features 

 of the animal. Even Dr. Simroth, whose profound knowledge of the 

 organization of the Arions is incontestable, feels compelled to remark 

 "that the species of the genus Arion are so difficult to distinguish anato- 

 mically, 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 

 nlmost 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 dorsum and a somewhat corpulent body ; rugose 

 SKIN ; an uncarinated and blunt tail ; foot 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 respira- 

 tory APERTURE near its right anterior end, and the genital orifice beneath. 



jVccording to Simroth, 'they originally possessed an ancestral lateral band 

 at eacli 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. 



The 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. 



The ALIMENTARY CANAL is more or less spirally triodromous, the coil being 

 held in position anteriorly by the aorta, as' is usual, and the stomach 

 TRACT being the most posteriorly extended ; the jaw is odontognathous' 

 or ribbed ; and the teeth of the radula cuspidate, with quadrate base 

 of attachment. 



The muscular system is quite different from that displayed by Limax 

 and by the Helicidic, in which the tentacular and pharyngeal muscles 

 unite posteriorly into a single band or possess a common base of attachment, 

 wliereas in Ariaii the pliaryngeal as well as each tentacular muscle have 

 their separate a,ud widely-distant places of attachment to the dorsal skin, 

 and constitute the section Trichoriza." This separation of the tentacular 

 retiactors is probably due to mechanical causes, the oblique strain tending 

 t(( pull ajiart tlie muscles, and as the soit degenerate shell no longer 

 aflords a firm attachment, the retractors have become fixed to the tough 

 dorsal iiiteguiuent or to the lung floor. 



1 Monog. !., p, '.Vil, f, .WP. 2 Monog. i., p. 314, f. 638. 



