420 PHYLUM MOLLUSCA. 
of the ctenidium type. The shell is usually in one piece ; but 
it is sometimes rudimentary or absent. The foot usually 
contains a mucus gland, and tends to be divided into three 
regions—the pro-, meso-, and meta-podium. There ts a singl: 
reproductive organ and genital duct. 
Branch A. STREPTONEURA 
In the torsion of the body one limb of the visceral loop crosses the 
other in a figure 8. 
Order 1, ZYGOBRANCHIATA 
The atrophy of the primitively left-side gills and nephridia is not 
carried out, or only partially, e.g. Halzotds (ear-shell) ; Azsszzella (key- 
hole limpet) ; Pated/a (limpet). 
Order 2, AZYGOBRANCHIATA 
The originally left gill and the originally left nephridium have been 
lost. Heart with single auricle, one gill, one nephridium ; operculum 
present. : Fa 
Periwinkle (Lzttorina), buckie (Buccznum, Fig. 222), dog-whelk 
(Purpura), Ianthina, and the majority of the marine Gasteropods 
with coiled shells, together with some fresh-water forms. The 
pelagic Heteropods are also included here :—Ad¢/anéa, shell well 
developed ; Cardnaria, 
-with small shell; P/ezo- 
trachea, with no shell. 
Branch B. EUTHYNEURA 
The visceral loop does not 
share in the torsion of the 
visceral hump. 
Order 3. 
OPISTHOBRANCHIATA 
The visceral loop is euthy- 
neural, as in snails; the 
single auricle lies behind the 
ventricle; the shell and 
mantle are often absent. 
A. Tectibranchiata. A 
shell is present, 
but may be rudi- 
mentary; there is 
Fic. 229.—A_ Ptleropod (Cymbulia a well-developed 
perontt), showing the wing-like expan- mantle fold and 
sions (pteropodial lobes) of the mid-foot. a single gill, e.g. 
