ANIMAL — EXTERNAL FEATURES. 
135 
Fig. 289. — Section through skin of Helix pomatia showing 
epithelial and subepithelial structure, highly magnified (after 
Vogt aud Yung), ep. epithelium; 1. small blood lacunze; 
m. muscle-fibres ; 7n. gl. mucus glands. 
are highly predacious, others purely phytophagous, hut the majority 
of our forms are quite omnivorous and devour almost any substances 
that come in their way. 
E.xternally the whole surface of the animal is formed by a layer of 
somewhat firm and elastic muscular tissue, overspread by cylindrical 
epithelial cells, which 
are often ciliated, more 
especially in the a(|uatic 
species, and in those 
parts not habitually 
concealed by the pro- 
tecting shell ; the 
ciliated areas in the 
terrestrial forms being more restricted in extent. Scattered over 
the whole external surface, but more densely aggiegated in certain 
definite parts are a number of neuro- 
epithelial cells forming the terminations 
of the organs of general and special 
sensibility in the mollusk. Intermingled 
with them are the outlets of numerous 
unicellular glands which secrete the more 
or less viscous mucosity which renders the teguments supple and moist. 
The body of the animal may be conveniently divided for study 
into four chief areas or regions, viz. : — The Cephalic, the Pedal, the 
Pallial, and the Visceral. 
The Cephalic or Oral region is the anterior portion of the body, 
and bears the mouth or oral aperture and most of the organs of 
special sensibility, with appendages of various kinds. 
The Pedal or Ventral region is typically a well developed abdominal 
protuberance of variable shape, formed by a differentiation and 
thickening of the cutaneous and muscular tissue of that region, and 
constitutes the locomotor organ or foot. It varies in structure, size 
and importance in correlation with the active or sedentary habits of 
the animal. 
The Pallial or Dorsal region is formed by a vascular expansion 
or duplication of the integument, called the mantle or shield, which 
partially or completely covers the more delicate parts or organs and 
hangs down around the body, the intervening space or cavity between 
the mantle and the body being kmown as the pallial chamber. 
Fig. 290. — Neuro-epithelial cell of 
Avion ater, highly magnified (after 
Boll). 
