296 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



ternal apertures of the reproductive organs may be found, 

 although they are so small that the section is not likely to 

 pass through them. They are a pair of minute openings, 

 on the sides of the upper portion of the abdomen, and are 

 so placed that the reproductive elements are discharged 

 into the cloacal tubes of the inner gills. 



2. In the section through the anterior part of the abdo- 

 men, notice : — 



(i.) The dark green liver which lies on the top and left 

 side of the abdomen. 



(ii.) The irregular cavity of the stomach, immediately 

 below and almost surrounded by the liver. 



a. The large openings of the bile ducts, upon its sides. 



(iii.) The large muscular foot, upon the free end of the 

 abdomen. 



(iv. ) The pedal ganglia embedded in the muscles of the 

 foot on the median line. 



XXVII. THE STRUCTURE OF THE LAMELLI- 

 BRANCHIATE GILL. 



The growing gills of an embryo and the simple gills of 

 such a form as Mytilus must be studied in order to 

 understand the highly complex gills of Unio or Anodonta. 

 In the embryo each gill is, at first, a row of tentacles, 

 growing out from the side of the abdomen into the mantle 

 cavity, and having their tips free in this cavity. 



As Cyclas gives birth to j'^oung throughout the a: hole 

 spring and summer, embryos of this genus may be pro- 

 cured without difficulty for the study of the early stages 

 of the gill. 



I. The examination of the gills of the Cyclas embryo. 



The various species of this genus are small fresh-water 



