SPECIAL ANATOMY. 205 
layer ; its epithelial cells are flattened, from three to six 
sided, granular, and with large, round nuclei. 
The muscular substratum (figs. A, B, C, 2) of the 
mucous lamina is composed of unstriped fibres, arranged 
transversely, obliquely, and longitudinally. It is inflect- 
ed outwards beneath the mantle, in Limax and Avion, 
to form the outer parietes of the pulmonary chamber. 
Between this portion and the mucous layer is placed the 
rudimentary testa (fig. A, 6). In Tebennophorus, it is 
inflected inwards (fig. B, 5) beneath the anterior portion 
of the mantle, to form the parietes of the pulmonary 
cavity. Its transverse fibres predominate within the 
tentacular, its longitudinal fibres, in the exterior pul- 
monary parietes of the testaceous genera, and especially 
accumulate on the outside of and parallel to the rectum, 
so as to serve as an efficient agent in the retraction of 
the collar, and an aid in the expulsion of matters from 
the rectum. 
Interior to the musculo-mucous investment of the body 
is a second covering, (figs. A, B, C, 3 ) which may be 
considered as a sort of peritoneum. It is a muscular 
membrane, and encloses the digestive and generative 
apparatus. It is usually pretty closely attached to the 
outer tegument, except in Tebennoplwrus carolinensis, 
in which the two are separated in all parts of the body, 
except above the pedal disc, where they are firmly 
blended together, as in all Gasteropoda. It forms the 
partition or diaphragm between the visceral and pulmo- 
nary cavities. This membrane is composed of transverse 
vol. i. 53 
