BODY SCULPTURE. 
205 
Fig. 405. — Sticcinea piitris (L.) 
X 3, Christchurch, Hams., showing 
the dorsal grooves. 
The Dorsal Grooves form the principal longitudinal gTooves or 
furrows ; they are a pair of parallel and 
deeply marked channels, separated by 
a row of elongate tubercles, and extend 
from between the tentacles, along the 
centre of the back, and may be con- 
tinued anteriorly upon the muzzle as the 
facial gTooves. 
Tlie Lateral Grooves are distinctly developed only in the testaceous 
forms, the right lateral groove arising near the pneumostome and 
reaching to the base of the tentacle near the genital opening, the 
left gToove when present having a similar course ; and the tubercles 
or rugosities of the body being usually noticeably larger and more 
oblong above the grooves than beneath, this difference in character 
being sometimes very sharply marked. These grooves, though now 
only serving to assist in the dissemination of the fluid mucus over 
the body, probably indicate the position of the ciliated conduit, 
which formerly conveyed the seminal fluid from the genital glands to 
the male organ ; they are 
especially developed in Teata- 
cella, arising together be- 
neath the anterior margin of 
the mantle, and traversing 
the sides of the body, giving 
off during their course a number of anteriorly directed furrows, whose 
ramifications are gradually obliterated in the general roughness of 
the integument. 
In the Pelecypoda the usually oval body, being entirely enclosed by 
the mantle lobes, is soft, smooth and somewhat strongly ciliated, hut 
the viscera have also become deeply sunk within the foot. 
In the Anisopleurous Gastropods, the dorsally protruding body has 
been subjected to a very remarkable and combined antero-ventral and 
lateral twisting or torsion, which renders it more compact and brings 
together the opposite ends of the alimentary canal. Such torsion is, 
however, not confined to Gastropods, or even to mollusca, as analogous 
processes have been observed to take place in Crustacea, Biyozoa, 
Echinoderms and other forms of life. 
The ontogenetic history shows this twisting in the Gastropods to 
occur before anything else which alters the primitive form, as the 
Fig. 406. — HyaUniahelvetica'^\\xm x 2, Horsforth, 
near Leeds, showing the lateral grooves. 
