THE PROSOMA OR CEPHALIC REGION. 
185 
The Morphology of the External Organs. 
Fig. 35G. — Head and Tentacles of 
a Ga.stropod, Helix aspersa X 2. 
THE HEAD AND ITS ORGANS. 
Tlie Cephalic region, which in both Gastropods and Pelecypods may 
be distinguished as the Prosoina, is bilaterally symmetrical, and arises 
in the emliryo from the upper surface of the Velar area, the size of 
which, according to Gegenbaur, is correlated with the differentiation 
of the head. In the adult this region 
is indicated by the position of the oral 
orifice, and in the Gastropoda is formed by 
the distinct and usually well-developed, 
somewhat cylindrical head and its appen- 
dages, placed at the anterior end of the 
body, and borne by a more or less 
evident and intervening constriction or neck, on the right side of 
which is placed the common genital orifice, or, as in the Streptoneures, 
may bear the muscular and non-invaginable male organ. The whole 
ceiihalic region is usually capable of being protruded beyond and 
comiiletely withdrawn beneath the mantle for i)rotection. 
In the Pelecypoda the distinct head of the assumed ancestor has 
become atrophied and lost, and there is therefore no specialized or per- 
ceptible head, its position being now only indicated outwardly by the 
transversely oval mouth, with its encompassing lips and 2)alps. 
The IMouth, or oral aperture with its labial aiipendages, is situated 
on the ventral surface of the head, or may, as in Strei)toneures, be 
placed at the extremity of a long and contractile rostrum. It 
originates, during the development 
of the embryo, as a simple invagina- 
tion of the ectoderm, termed the 
Stomodieum, which meets and joins 
with the mesenteron or mid-gut. 
In those cases where the blastopore 
or orifice of primitive invagination remains permanently open to form 
the mouth, a prominence or wall becomes developed which surrounds 
or encompasses it. 
In the mollusca, as in other Invertebrates, the mouth is only fitted 
for the inception of food, the respiratory orifice, which in some of the 
higher animals is in a measure confounded with the oral opening, 
being quite removed from its proximity. 
Fig. 357.^ — Mouth of Arion atcr X 10 
(after Moquin-Tandon), showing also the jaw, 
the upper and lower lips, and the lobulate 
organs of Semper on the upper lip. 
