536 ScHARFF — On the Slugs of Ireland. 



situated close to it. There never is a well-marked keel except in young specimens. 

 The intestine has four convolutions. 



The most apparent characteristic by which this genus may be distinguished 

 from the preceding ones are the presence of a caudal gland, and the fact of the 

 respiratory opening being situated in front of the middle of tjie mantle. 



The caudal gland is easily visible externally (Plate LVI., fig. 10). Its opening 

 is situated at the very end of the body, and is triangular in shape ; the base of the 

 triangle being directed towards the head. 



Another, perhaps, less noticeable feature is that of the reproductive pore, 

 which lies quite close to the respiratory opening, whilst we have seen that in the 

 other genera it is situated near the tentacles. 



The mantle, or shield, in Avion is granvdated as in the genus Amalia. 



All the species of Avion are altogether broader in shape than any of the slugs 

 hitherto considered. 



There never is, in Avion, a well-marked keel, and though we may, as in young 

 Avion hortensis and A. bourguignati, have slight indications of one, it is nothing like 

 what obtains in Limax, Agriolimax, or Amalia. The wrinkles are generally more 

 jDrominent in Avion than in the other genera. 



As regards the internal structure of the genus Avion, I may mention in the 

 first place that there is no solid shell, but a soft mass of calcareous granules, 

 which, in some of the smaller species, may be somewhat firmer than in the larger 

 ones. Simroth (38) has pointed out, that whilst Limax, Agviolimax, and Amalia 

 utilize their male end-organs during copulation, in Avion the female end-portions 

 of the reproductive organs are used as penes. What is generally looked upon as the 

 penis in Avion is no such thing, there being no retractor muscle to it. The enlarged 

 end portion of the sperm-duct is used for the preparation of the spermatophore 

 (sperm-case), and Simroth (38) has apjilied to it tlie term " Patronenstrecke " 

 (cartridge-portion). 



There are four convolutions in the intestine, the first being the largest. 



Synopsis of the Irish Species of Arion.* 



A. — Margin of foot with transverse striae = A. atev, A. subfuscus. 



I. — "Wrinkles keeled and elongated = A. afcr. 



II. — Wrinkles flat and short = A. subfuscus. 



* It is very difiScult to give good external distinctions for the species of Arion, but the above will 

 be found fairly practical, if it be kept in mind that wrinkles are a variable feature and that slugs nuist 

 be compared under similar atmospheric conditions. 



