YII 



MOLLUSOA—MUSGULATUBE 



121 



1. Prosobranehia. 



The columellar muscle is here always developed in its typical form. 

 It is attached at one end to the columella in the last coil of the shell, 

 and at the other to the operculum, which lies on the dorsal side of the 

 metapodium. 



A few Prosobranehia, such as most Fissurellidce, Haliotidce, and Docoglossa, use 

 theu- foot chiefly as a sucker for 

 attaching themselves to some firm 

 surface. Tliese forms have no 

 operculum. The columellarmuscle 

 descends vertically into the foot, 

 and by its contraction presses the 

 shell against the surface to which 

 it is attached. In Ealiotis (Fig. 

 105), the ear-sliaped shell of which 

 is coiled, this muscle is cylindrical 

 and is very highly developed ; it 

 runs somewhat to the right of the 

 median plane, at right angles to 

 the pedal disc, thereby pushing the 

 mantle carity and the viscera to 

 the left. In many FissureUidco 

 and tlie Docoglossa, the shell has 

 become cup - shaped and sym- 

 metrical ; the columellar muscle, 

 Avhich is very much shortened, 

 descends direct from the inner 

 surface of the shell to the foot, 

 and is no longer cylindrical. The 

 whole muscle has the form of a 

 short truncated hollow cone, open 

 anteriorly, which is attached to 

 the shell by its upper horseshoe- 

 shaped sectional surface, and, by 

 its base of the same shape, to tlie 

 sucker-like foot. The viscera are 

 contained in its hollow axis (Fig. 

 106). The same arrangement 

 occurs in all cases where the shell 

 is flatly conical, cup- or plate-shaped, as in the Hipponycidce and tlie Capiilidm 

 among the Monotocardia. 



Heteropoda. — In this order, in which the atrophy of the sliell, the transformation 

 of the foot, and the gradual obliteration of all resemblance to a Gastropod can be 

 traced, step by step, the musculature deserves special attention. 



In Atlanta, where the head and foot can still be completely withdrawn into the 

 shell, the columellar muscle retains its typical form. It descends from the shell, 

 dividing into three strands ; the strongest median strand stretches out into the fin and 

 the sucker, the posterior into the operculum-bearing metapodium, and the anterior, 

 which is the smallest, into the head and snout. 



Fig. 10.!i. — Haliotis, from above, after removal of the 

 shell, the mantle, and the entire dorsal intngument (after 

 Wegmann). t, Snout ; s and %>, salivary glandy ; pi, lateral 

 poclvets of the ojsophagus ; i, mid-gut ; a, CESophagus ; r, 

 rectum ; e, stomach with caecum (c) ; 7i, digestive gland 

 (liver), its right-hand portion which lies next the large 

 columellar muscle (m) is covered by the genital gland. 

 A fringed epipodium encircles the body. 



