1922. Xo. 5. ARIOXIDAE OF NORWAY 



Family Arionidae Gray. 



Diagnosis. Most genera without external shell, the rudiments of which is then concealed 

 under the shield covering the anterior part of the body, kidney circular, enclosing the pericar- 

 dium, the reproductory organs often without a penis, the columellar muscle generalh' di\-ided. 



Several groups of gastropoda have in common a reduction of the 

 shell, each presenting to us a series of species which illustrate this 

 successive reduction. Amongst the air-breathing molluscs, too, there are 

 families with such parallel evolution, one of which is the Arionidae. The 

 American Biimcya has an auricle-shaped external shell, which partly covers 

 the visceral mass but which cannot conceal the whole animal. In another 

 American genus, HcmpJiillia, however, the folds of the mantle partly 

 conceal the shell, which is quite destitute of coils, while its central part still 

 remains visible. But most genera have no visceral mass projecting on the 

 dorsal side. The latter has, so to speak, sunk down into the foot, and the 

 covering shell is fully concealed under the united folds of the mantle which 

 form a shield on the foremost part of the back. The shell thus enclosed 

 in a sac degenerates, and finally, in the genus Arioii, is only represented 

 by tiny calcareous granulations. 



Also with regard to other anatomical characters, the genera often 

 exhibit différences ; yet relationship is clearly shown by the peculiar form 

 of the kidney \nepliridiutn\. In the members of this family it encloses 

 the pericardium in the shape of an elliptical or almost circular ring. In 

 other pulmonates, however, it is represented by an unpaired sac between 

 the rectum and the pericardium. 



The reproductive organs, which are of the greatest s\-stematic 

 importance, offer a multitude of forms. A peculiar feature is the frequent 

 absence of a penis. 



The shape of the columellar muscle [iniiscuhis cohtmellaris) '\s dXso 

 of interest. In the shell-bearing pulmonates this muscle is powerfully de- 

 veloped and gives off branches to the foot, the pharynx, the head and 

 tentacles; it is affied to the shell, into which it retracts the animal. The 

 less differentiated Arionids have retained this coherent retractor muscle, in 

 others, as in our Arions, it is divided into several components. 



As regards distribution, the Arionidae are found in all continents, 

 but especially in the northern hemisphere. The West of North America has 



