366 



PHYLUM MOLLUSC A. 



ccelom usually persists as the pericardium at least (Fig. 

 1 8 1, s.}, and communicates with the exterior through the 

 nephridium or nephridia. The rest of the cavity of the body 

 is hcemoccelic. The vascular system is almost ahvays well 

 developed, but though part of the rirculatio?i is in most cases 

 through ill-defined spaces or lacunce these are quite separate 

 fro?n the ccelom ; the heart typically consists of a ventricle and 

 two auricles. Respiratory organs are most typically repre- 

 sented by gills or ctenidia, consisting of an axis attached to the 

 body and bearing lamellce, but the gills may have simpler 

 forms, or may be absent^ and in the terrestrial snails the 

 mantle cavity is adapted for ferial respiration. At the base 

 of the gills there is generally an olfactory organ or osphradium. 

 The sexes are separate or united. There are tivo common 

 larval stages, the Trochosphere, which resembles the same 

 stage in some Annelids, and the more characteristic Veliger 

 (Fig. 182) ; but the development is often direct. The Mollusca 

 form a very large phylum, with much diversity of habit. 



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

 terrestrial (pulmonate) Gasteropods. 



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

 larger edible snail (H. pomatid], which is rare in England 



FIG. 183. Roman snail (Helix pomatio]. 



Note shell covering visceral hump; p-ctp., pulmonary aperture 

 (including anus and opening of preter) ; f., the foot ; g.ap., 

 genital aperture ; ;;/., mouth ; <.'.,eye on long horn ; sJi., one of 

 short horns. 



but abundant on the Continent, serves as a convenient type 

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



