130 OTHER MOLLUSKS 



infected twenty-four to forty-eight hours previously may be provided 

 for examination on the same day as the foregoing. Record the condi- 

 tion of these glochidia and make figures. Glochidia of Lampsilis 

 subrostrata remain on the sun-perch, Lepomis pallidus, for a period of 

 ten to thirty days, according to the temperature, and then drop to the 

 bottom as young mussels which crawl actively about (Fig. 60 D). 

 They may then be seen with the unaided eye and collected without 

 difficulty. 



OTHER MOLLUSKS 



Exercise 1. — ^The Pond Snail. 



(a) For the following study the French snail. Helix, will be found 

 very satisfactory, but the common pond snails, Physa, Planorbis, or 

 Lymnxa, may be used to advantage. Observe the shell. Is there any 

 division into valves? What is the general form of the shell? How 

 many turns does it make? Each turn is known as a whorl. How do 

 these whorls vary in size? Compare several specimens of the same 

 species as well as several different species. If the coils turn to the 

 right, the shell is dextral; if to the left, it is sinistral. Is the coiling 

 loose or close, flat or conical? The apex of the shell is the oldest 

 part. The wide opening of the shell is its mouth, or peristome. What 

 is its shape? Is the margin smooth or toothed? Explain. One side 

 of the peristome is drawn out into a spoutlike process in some species. 

 What is its use? Do you find in any of the species an oval plate 

 closing the opening? Such a structure is called the operculum. To 

 what is it attached? Can you explain its use? The whorls make a 

 line where they come in contact; this is the suture. Is the suture a 

 smooth or an irregular line? Explain the lines that run parallel to the 

 edge of the peristome as lines of growth. Is there any variation? 

 The whorls coil around a central axis, the columella, as can be seen 

 in a large shell that has been sawed lengthwise or from which the 

 outer parts of the whorls have been broken. 



(b) Fasten a specimen in a watch glass with wax so that the 

 peristome of the shell is uppermost and barely covered with water. 

 Study under the microscope. Or allow a small snail to become at- 

 tached to a slide which may then be inverted, placed across the top of 

 a watch glass, and examined under the microscope. How is the gliding 

 movement effected? What is the shape of the foot and its relation to 

 the rest of the body and to the shell? Can you observe cilia? Are 

 there progressive waves of motion? The anterior region of the foot is 

 the propodium; the posterior is the metapodium; and the mid-region 

 is the mesopodium. Are these regions sharply marked off from one 



