6 THE CHICAGO ACADEMY OF SCIENCES. 



rate, the lateral margins being pointed. The tentacles are large, tri- 

 angular and placed on top of the head just above the junction of the 

 vela area with the foot. The black eyes are placed on small swellings 

 at the inner base of the tentacles. The mantle fits closely to the aper- 

 ture of the shell, but does not extend beyond the margin of the outer 

 lip or the edge of the reflected inner lip. The respiratory orifice is placed 

 near the posterior angle of the shell, where a portion of the mantle 

 is modified to form a so-called siphon which is capable of great ex- 

 pansion when the animal is taking in air. The pulmonary cavity fills 

 about half of the body whorl and is usually colorless and transparent 

 as seen through the shell. The heart may be distinctly seen through 

 the thin tissues and its pulsations easily counted. 



The heart pulsations of several specimens are tabulated below : 



Lymn&a stagnalis 37- 48 per minute. 



Pseudosuccinea columella 53-60 " " 



Galba obrussa 150-155 " " 



Galba humilis modicella - 140-146 " " 



Galba caperata 129-133 " " 



Galba palustris 80- 81 " " 



The smaller species appear to have a higher pulse than the larger 

 ones, stagnalis, the largest species, having 37-48 pulsations, while 

 obrussa, one of the smaller species, has 150-155 pulsations per minute. 



There are no grooves on the body as in the helices, and the texture 

 is smooth instead of being granular. The shell is carried, as a rule, 

 in a direction parallel with the foot, although it may at times be swung 

 around until it rests obliquely or even at right angles to the foot. ( Fig. 

 H.) In color the animals vary from yellowish, flecked with whitish 

 or yellowish spots, to jet black. Frequently the black dashes, spots 

 or blotches of the mantle may be seen through the transparent shell, 

 causing the latter to appear mottled. The foot and tentacles vary to 

 some extent among the different species, the variation being in the 

 length of these organs. 



3. GENERAL INTERNAL ANATOMY.i 



If a large specimen of Lymnaea be opened on the back and the 

 organs be carefully separated and spread out (as in plate I), the 

 relations and characteristics of the various anatomical features may 

 be easily made out. In the plate, the genitalia are turned to the left, 

 while the digestive and respiratory systems are turned to the right. 



J This topic is treated in a somewhat superficial manner, the purpose being 

 simply to point out the more obvious characteristics of the various organs. 

 The common American Lymnaeid, Galba reflexa Say, has been selected for this 

 purpose. 



