382 PHYLUM MOLLUSC A. 



striped fibres, or fibres with unstriped fibrils twisted in a 

 spiral. A portion of the true body cavity or cozlom usually 

 persists as the pericardium at least (Fig. 205, s.), and 

 communicates with the exterior through the ?iephridium or 

 nephridia. The rest of the cavity of the body is hcemoccelic. 

 The vascular system is almost always well developed, but 

 part of the circulation is in most cases lacunar ; the heart 

 typically consists of a ventricle and two auricles. Respiratory 

 organs are most typically represented by gills or ctenidia, 

 consisting of an axis 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 is 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 is often direct. 



First Type of MOLLUSCA. The Snail (Helix), one of the 

 terrestrial (pulmonate) Gasteropods 



Habits. The common garden snail (H. aspersa), or the 

 larger edible snail (H. pomatia), which is rare in England 



n 



p.ap 



FIG. 207. Roman snail (Helix pomatia}. 



Note shell covering visceral hump ; /./., pulmonary aperture 

 (including anus and opening of ureter) ; f. , the foot ;./., 



one of 



shell covering visceral hump ; /./., pulmonary aper 

 including anus and opening of ureter) ; f. , the foot ;. 

 enital aperture ; w., mouth ; <?., eye on long horn ; s.A., on 



geni 

 shor 



t horns. 



but abundant on the Continent, serves as a convenient type 

 of this large genus of land-snails. They are thoroughly 



