No. 471] ANATOMY OF ACM^A TESTUDINALIS 181 



' The dorsal part of the body is in the main developed into the 

 convex visceral mass, but just above the anterior part of the foot 

 it suddenly contracts into the neck (Fig. 2) and this, passing for- 

 ward and slightly enlarging, gives rise to the head which curves 

 ventralward and ends in the so called muzzle, thus bringing the 

 mouth to lie flush with the foot. The head consists of a thin, 

 muscular wall which in the region of the muzzle is fused with the 

 pharyngeal walls but farther back is separated from them bv a 

 large blood sinus. 



The visceral mass contains the digestive tract, blood vascular 

 system, reproductive glands, and nephridia. It is covered by a 

 green epithelium which immediately underlies the shell and may 

 readily be brushed away. This being done, parts of all the above- 

 mentioned organs may be made out through the thin body wall 

 though they can be seen somewhat more readily in a specimen 

 preserved in Gilson's fluid or formalin. 



In such a preparation (Fig. 4) one notices first the band com- 

 posed of pallial and columellar muscles; it is divided into a series 

 of fascicles by blood vessels which cross it. External to the nniscle 

 ring is the mantle, fringed with its tiny tentaclrs and inaikc.l on 

 the edge with a band of pigment whose alternations of Yv^ht and 

 dark tint have a general correspondence with tlic H^ht and dark 

 radial markings of the shell. In this region the mantle is thick- 

 ened by the presence of a mass of nnicellnlar glamls of uncertain 

 function. Just internal to the pigment band is a zone, often con- 

 tracted to extreme narrowness, which represents the thin, non- 

 glandular part of the mantle. Internal to the anterior curve of the 

 pallial muscle and between the ends of the columellar muscle is a 

 pellucid space, the roof of the nuchal cavity, in wliicli may be noted 

 traces of a blood plexus and through which the ontlines of the 

 ctenidium and the head may be more or less clearly x-cn. 



Turning now to the visceral mass we note just internal to the 

 columellar muscle and pericardium (see below) the edges of the 

 generative gland; the bulk of the organ lies in the ventral part of 

 the body directly above the foot, but its margins, especially the left 



