SPECIAL ANATOMY. 005 
pressa, it is wholly enveloped in a sheath derived from a 
tubular onset from the duct of the generative bladder. 
The muscular tunic of the penis is thick and strong. 
The internal lining mucous membrane usually presents a 
number of large rugae, longitudinal and oblique ; fre- 
quently there is but a single, large, longitudinal fold, as 
in H. alternata, H. albolabris, etc. At the point of en- 
trance of the vas deferens there is generally one or two 
pendant, valve-like folds of the lining membrane. 
In H. albolabris, H. tridentata, and M. multilineata, 
the surface of the membrane is everywhere distinctly 
papillated ; in the others it is smooth. 
The penis of H. cellaria on the outside presents a row 
of minute, round, glandular bodies. 
The retractor muscle is in all cases, except in H. soli- 
taria, inserted into the summit of the penis, or into the 
vas deferens near its termination in the latter. In the 
excepted case, it is inserted into the side of the penis, 
above its middle. In H. multilineata there are some 
accessory fibres passing from the latter to the prepu- 
tium ; in R. profunda to the base of the penis ; in H. 
albolabris, tridentata, etc. from the vas deferens to the 
preputium. The penis joins at its base the cloaca. 
The ovary has the same general form and color as in 
slugs, but rarely presents any thing more than a trace of 
lobuli, usually having a uniform, homogeneous appear- 
ance. 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 blad- 
vol. 1. 58 
