104 PNEUMONOBRANCHIATA. 



Fam.l. ARIONIDtE. 



Head and tentacles retractile* into the skin, which 

 covers them as a sheath, being drawn into the ca- 

 vity of the body ; the end of their tail is as it were 

 truncated, and furnished with a gland on its upper 

 edge ; the respiratory cavity is in the front of the 

 body, with the hole in the front of the mantle's 

 edge ; and the orifice of the generative organs is 

 placed on the right side, near, or immediately 

 under, the respiratory aperture, (p. 103. f. 1.) 

 The shell is presented in very different degrees of 

 development in the different genera, — it is very 

 rudimentary in the only English one. 



6. 1. Arion Ferus. (Land Soles.) 



Body elongate, lanceolate, united its whole length to the 

 foot ; mantle shield-like, anteriorly ovate, granular ; 

 the .orifice of the generative organs is immediately 

 under the respiratory aperture. Shell distinct, ob- 

 long, sometimes only spongy, or only a few granules 

 in the subtance of the mantle. 



* Shell none, or hemispherical and spongy* 



9, 1. Arion ater. Black Arion. Tentacles black; the 



* Ehrenberg proposes to call the tentacles of snails tentacula> 

 and those of pond snails, which do not bear eyes, vibracula. 



