While Professor Parker had been speaking, Harry, 

 who was the most observing member of the quartette, 

 and who had been carefully examining a darn with 

 one valve removed, exclaimed, "What's this thing 

 in the upper part of the animal which pulsates so 

 regularly ? ' 



Professor Parker took the specimen and looked 

 at it for a few moments, saying, " This is the clam's 

 beating heart. It is composed of a central ventricle 

 and two lateral auricles which are contained in the 

 pericardium, this being the upper cavity which you Crogs gection of 

 saw in my drawing of a section of the clam. There fresh-water clam pass- 



mg through the heart. 



are several large blood-vessels which carry the blood A, paiiial, or branchial 



chamber; a, auricles; 

 cl, cloacal chambers of 



tine ; ig, inner gills ; k, 

 kidneys ; m, mantle ; 

 og, outer gills ; p, peri- 

 cardium, or chamber 

 the heart ; 



through different sized veins into every part of the 

 body. The throbbings which we see in this speci- 

 men are the pulsations of the ventricle pumping the 

 blood into the veins. After flowing through the 

 body and becoming loaded with carbonic acid gas, 

 the blood passes to the gills, where it discharges the 

 poisonous gases into the pallial chambers, absorbs fresh oxygen from 

 the gills, and enters the two auricles, one auricle being placed over each 

 pair of gills, to be again pumped through the body." 



Harry was asked to count the pulsations of the heart and he did 

 so, finding sixteen per minute. This was an interesting occupation, 

 and we each counted in turn, finding the records to vary from four- 

 teen to sixteen. 



George, who from his habit of asking endless questions had been 

 dubbed the interrogation point, asked how the young clams grew. 



This question led the Professor to explain to us 

 the wonderful method of growth in the fresh-water 

 clams. 



" The gills of the fresh-water clams are modi- 

 fied to form marsupia, or pouches, and in these 

 the young clams develop from the egg until 

 they attain a certain size and shape called a 



Anterior viewer 'Glo- 



chidium" of Anodonta, glochicliiim. The shell, mantle, and muscles 



inclosed in the eggshell. , , -. , ,. . 



b, byssus; bg, byssus are developed, but the digestive organs are not 

 hook"; is, FSfvalVe of ^rmed until six months or a year afterward. The 

 shell; m, posterior ad- animal is inclosed in a thin eeesliell in which 



ductor muscle ; rs, right 



valve of shell; s, setae; are enveloped the embryonic shells of the animal. 



v, velum. Greatly mag- rp, . , , . . . 



nified. (After Brooks.) 111686 are united by a hinge, which is so elastic 



7 



