GASTROPODA — STREPTONEURA AND EUTHYNEDRA. 
143 
From this archetypal form our various species have been evolved in 
the two well-marked divergent lines, Gastropoda and Pelecypoda, 
characterized by and receiving their names from the modifications 
the foot has undergone. 
The Gastropoda, pursuing an active and more or less aggressive 
mode of existence, have developed the most varied and numerous 
forms and are the only group of mollusks containing species organized 
for the respiration of free air ; they have also retained and variously 
developed the sole-shaped locomotor foot and the distinct head of 
Fig. 302. 
Fig. 303. 
Fig. 302. — Schematic figure showing the arrangement of the internal organs in Helix aspersa ns 
typifying the Euthyneura. ^ Fig. 303. — Schematic figure showing the arrangement of the internal 
organs in the Azygobranchiate Streptoneiira (after Lang). 
a.g. abdominal ganglion ; b.g-. buccal ganglia ; c.£^. cerebral ganglia ; p.g-. pedal ganglia : pLg. 
pleural ganglia ; p.v.g. pallio-visceral ganglia, representing the long twisted nerve cords and 
ganglia of the Streptoncures ; sb.^. subintestinal ganglion ; sp.g. supraintestinal ganglion ; e. eye ; 
os. osphradium ; a. anus ; oe, oesophagus ; k. kidney or nephridium ; r. rectum \g.o. genital orifice ; 
p. male organ ; s.f. seminal furrow ; br. branchia ; pc. pericardium, enclosing the heart ; s. siphon ; 
r.c. lung chamber, showing blood plexus \f. foot ; hyp.gl. hypobranchial gland ; op. operculum. 
their assumed ancestor, and acquired or elaborated a very remark- 
able prehensile and aggressive oral mechanism adapted for the seizure 
and comminution of food, but owing to the une({ual pressure or strain 
upon the body, caused by the development of an univalve shell, they 
have lost the bilateral arrangement of many of their internal organs. 
The various species have not however developed pari pasim, and 
in accordance with the differences now exhibited are distinguished as 
Streptoneiira and Euthyneura, the former characterized by the per- 
