382 PHVLUM MOLLUSCA., 
striped fibres, or fibres with unstriped fibrils twisted in a 
spiral. A portion of the true body cavity or celom usually 
persists as the pericardium at least (Fig. 205, S.), and 
communicates with the exterior through the nephridium or 
nephridia. The rest of the cavity of the body is hemoceltc. 
The vascular system is almost always well developed, but 
part of the circulation ts in most cases lacunar, the heart 
typically consists of a ventricle and two auricles. Respiratory 
organs are most typically represented by gills or ctentdia, 
consisting of an axts attached to the body and bearing lamella, 
but the gills may have simpler forms, or may be absent, and 
in the terrestrial snails the mantle cavity 1s adapted for 
aerial respiration. At the base of the gills there is generally 
an olfactory organ or ‘osphradium. The sexes are separate 
or united. There are two common larval stages, — the 
Trochosphere, which resembles the same stage in some 
Annelids, and the more characteristic Veliger (Fig. 206) ; 
but the development ts often direct. 
First Type of Motiusca. The Snail (/e/ix), one of the 
terrestrial (pulmonate) Gasteropods 
Habits.—The common garden snail (47. aspersa), or the 
larger edible snail (A fomatia), which is rare in England 
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Fic. 207.—Roman snail (//elix pomatia). 
Note shell covering visceral hump; Z.a¢., pulmonary aperture 
(including anus and opening of ureter); 7, the foot; g.ap., 
genital aperture ; #., mouth ; ¢., eye on long horn; s.4., one of 
short horns. 
but abundant on the Continent, serves as a convenient type 
of this large genus of land-snails. They are thoroughly 
