](i THE CHICAGO ACADEMY OF SCIENCES. 



light and their visual powers must be developed to a considerable de- 

 gree. The sense of smell is centered in the ganglion known as the os- 

 phradium (25). The sense of hearing is centered in the paired ganglia 

 known as otocysts (31). These equilibrating organs consist of a sac 

 filled with a fluid secretion, in which are suspended a greater or less 

 number of calcareous bodies called otoliths, which are kept in a constant 

 state of vibration by means of minute cilia. As many as a hundred 

 otoliths have been found in adult Lymnseas and as few as seven in 

 the young individual. In Lymnaea the otocysts are connected with 

 the cerebral ganglia by a short nerve. It is not known definitely 

 whether or not a special sense of taste is present, although there is 

 no reason for doubting the existence of this sense, at least to a limited 

 degree. 



f. THE RESPIRATORY SYSTEM. Plate I. 



The respiratory cavity occupies nearly half of the last whorl 

 (plate I, 39). It is longer than wide and is supplied with a network 

 of blood vessels. These connect with the branchial artery or pulmon- 

 ary vein (37), which conducts the aeriated blood into the auricle and 

 from this organ into the ventricle (34, 35). The respiratory cavity 

 is situated on the right side of the body and is closed by a lobe of 

 the mantle, which forms a sort of siphon. 



g. THE RENAL SYSTEM. Plate I. 



The kidney or renal organ (excretory) in Lymnaea is of ex- 

 traordinary size, occupying the greater part of the respiratory cavity. 

 It is somewhat pyriform in shape, very wide, and in its natural position 

 appears like a large bag. It may be divided into two parts, a glandular 

 portion (plate I, 40), which appears as a long yellowish organ which 

 is filled with many small, rounded bodies, easily forced out by pressure 

 and a second portion, appearing grayish-black in color, which is plenti- 

 fully supplied with blood vessels (plate I, 38). The pulmonary vein 

 lies between these two lobes. The ureter is very short and wide, and 

 passes in a straight line to the anterior border of the respiratory cavity, 

 where it has its opening not far from the rectum. (Plate I, 41.) 



The direct passage of the ureter without flexure is another evi- 

 dence of the antiquity of the fresh-water pulmonates, showing that 

 they probably preceded such land forms as Partula and Achatinella. 

 which also have the direct type of ureter, and which are less modified 

 than the higher group (Sigmurethra) with the sigmoid ureter. The 

 kidney in Lymnaea is very uniform in the different species, varying 



