On Steno cypris Chevreuxi. 19 



immediately above the adductor muscle of the shell a 

 cylindrical cæcum, the so-called liver-sac. The latter 

 immediately penetrates between the 2 lamellæ of the valves, 

 extending in each of them diagonally to the hind extremity 

 (see fig. 4). These cæca are lined internally with large 

 secretory cells, and generally exhibit a more or less brownish 

 hue. In the living animal at times a rhythmical peristaltic 

 movement of these cæca may be observed, which caused 

 Fischer to regard them erroneously as hearts. The posterior 

 part of the intestine, the gut, is also rather capacious, and 

 extends through the abdominal division of the body, 

 terminating in a short rectum, which debouches immediately 

 in front of the insertion of the caudal rami. It is generally 

 found to be filled with a dark matter, whereas those of 

 the stomachal part always appear considerably lighter. The 

 walls of both the stomachal and abdominal parts of the 

 intestine are strongly muscular, and during life both parts 

 are seen to perform rather energetic movements. 



The circulatory system is very imperfect. There are 

 no blood-vessels whatever, and not even the slightest trace 

 of a central organ, a heart, is to be detected. The colour- 

 less blood is contained in the interstices between the inner 

 organs, and is put in motion, it would seem, chiefly by 

 the movements of the intestine. 



The respiratory system is represented by 3 pairs of 

 delicate, setiferous lamellæ appended to the mandibular palps 

 and the 2 pairs of maxillæ. These lamellæ, which in the 

 living animal are in rhythmical vibratory motion, are there- 

 fore generally termed branchial appendages. It is, how- 

 ever, most probable, that these appendages are in fact 

 only subservient to the respiration, and that the latter 

 process (the exchange of gases) takes place chiefly in the 



