144 
HELIX — EXTERNAL FEATURES. 
sistence of tlie tor.-iioii of the visceral commissures, and secondarily 
by the less coniiilete degeneration in the group generall)" of the 
primitively left organs of the pallia! complex ; and the Euthyneures, 
which have untwisted nerve cords and a greater concentration of the 
visceral nerve centres, with a more complete atrophy of the organs of 
the originally left side, perhaps due to the partial detorsion which the 
visceral sac has probably undergone. 
Helix aspersa or //. pomut'Kt, which may he accepted as repre- 
sentative of the Gastropoda, has a bilaterally symmetrical external 
aspect, with a compact hut elongate body, covered with an external 
cuticle composed of distinctly nucleated epithelial cells, distinctly 
ciliated on certain portions of the body, and usually more or 
less darkly pigmented and tuherculate, hut most strongly so in the 
anterior and dorsal regions, the pigmentation and rugin becoming less 
])ronounced and striking as the foot or the mantle are api)roached. 
Fig. 301 . — Helix nsf>ersa Mull., 
Kettering, Northamptonshire, collected by Mr. C. E. Wright, 
Showing the arr.ingenient of the tentacles and labial lobes, the genital furrow, the foot margin, 
the right and left lobes of the mantle separated by the partially expanded respiratory orifice, and 
the approximate position of the termination of the alimentary canal. 
Jldlr ((•<iier!<ii has a well develojR'd, distinct and hluntly roundeil head 
])laced at the anterior end of a narrow, oblong body, and covered 
with rounded and separate tubercles which show no trace of the 
symmetrical surface canals or facial grooves ; there are four elongate, 
retractile, divergent and finely granulated tentacles, the upper or 
posterior ])air hearing the eyes at their summits, and being about four 
times the length of the lower or anterior i)air. In other groups there 
may, however, he two tentacles only, hearing the eyes either at the 
ai)ex or at the base according to the genus. The elongate body is 
covered externally by numerous tubercles or rugosities forming 
about sixteen more or less ol)liipiely disposed longitudinal rows 
at each side of the noticealdy ])aler and more elongate row of 
mid-dorsal tuhei’cles which separate the two dorsal grooves or 
furrows : the ill-defined lateral or genital groove extends on the 
