254 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



oblique ; frequently there is but a single, large, longitudinal fold. At the point 

 of entrance of the vas deferens there is generally one or two pendant, valve- 

 like folds of the lining membrane. 



In some species the surface of the membrane is everywhere distinctly papil- 

 lated ; in the others it is smooth. 



The retractor muscle is, in almost all cases, inserted into the summit of the 

 penis, or into the vas deferens near its termination in the latter. In the ex- 

 cepted cases it is inserted into the side of the penis, above its middle. In 

 multilineata there are some accessory fibres passing from the latter to the 

 prepuce ; in prof undo, to the base of the penis ; in albolabris, tridentata, etc., 

 from the vas deferens to the prepuce. The penis joins at its base the cloaca. 



The ovary has the same general form and color as in slugs, but rarely pre- 

 sents anything more than a trace of lobuli, usually having a uniform, homo- 

 geneous appearance. The oviduct does not differ from that of the slugs. Its 

 neck is usually narrow, and of variable length, and is joined at the lower part 

 by the duct of the genital bladder, to form the vagina. In multilineata, the neck 

 is long, dilated at its lower part, and strongly muscular, and its internal surface 

 presents a number of longitudinal ruga?. In prof undo andfuliginosus,it is long, 

 cylindrical, and strongly muscular. 



The genital bladder, constantly existing, presents considerable variation in 

 the form, size, and length of the duct. It is generally subrotund, oval, or 

 pyriform in shape, and large. The duct is sometimes wide, as long as the ovi- 

 duct, and dilated at its lower part. In other cases it is as long as the oviduct, 

 and narrow, or it is rather more than half the length of the oviduct. In the 

 remaining cases, generally, the bladder reclines upon the lower part of the 

 prostate gland, and its duct is about the length of the neck of the oviduct. In 

 some it does not reach the prostate gland, and so gradually passes into its duct 

 as to be a mere long, cascal tube. In others the duct of the bladder is as short 

 as that of Arion. Usually, the surface of the bladder is smooth; in profunda 

 and exoleta it is transversely folded ; in fuliginosus it is regularly, longitudi- 

 nally folded. In multilineata the duct of the bladder at its termination dilates, 

 and is strongly muscular. In solitaria the lower third is dilated. In fuliginosus 

 and profunda it is strongly muscular, the greater part of its extent. In albo- 

 labris, palliata, tridentata, etc., it is dilated to the size of the bladder, is strongly 

 muscular, and internally presents a number of regular, longitudinal folds, some- 

 times undulated at the sides, extending to the lining of the bladder in the form 

 of line-like plicfe. In ligerus, intertextus, gularis, and suppressus, an offset from 

 the duct of the bladder passes down, and encloses the penis, dart sac, and cloaca. 



The vagina, or common duct of the oviduct and duct of the genital bladder, 

 holds no correspondence with the length of the penis; it is always shorter, 

 usually not more than one third the length, and is also narrower. In fuliginosus 

 it is surrounded by a thick, glandulous body. 



In ligerus, interlextus, gularis, and sitppressus there exists, opening into the 

 cloaca, a curved, cylindrical, strongly muscular dart sac, longer and narrower 



