174 Mr. L. E. Griffin on the 



labial lolje and Van der Hoeven's organ are homologous 

 organs. 



Tliere are about sixteen lamellae in the group in the centre 

 of the inferior labial lobe. The group is separated at the 

 median line into two iialves, the lamella of each side facing 

 each other. The largest lamellaB are at the centre of the 

 group, the smallest at the exterior. The lamella^, are marked 

 upon both surfaces by grooves parallel to tlieir bases. A 

 nerve showing some traces of ganglionic enlargements runs 

 to the tip of each lamella. The nerves of the lamellas of 

 each half of the group unite. The trunk thus formed unites 

 with the nerves of the tentacles of the lobe of its own side and 

 the common trunk enters the pedal ganglion near the median 

 line. The tentacles of each lobe are largest near the outer 

 end of the series. They grow smaller and smaller as the 

 median line is approached ; those nearest the median line are 

 frequently so small as to be scarcely visible. At this point 

 it is possible to find a complete series of gradations between 

 the lamellse of the median organ and the tentacles. The 

 structure of the lamellee confirms the suspicion that they are 

 modified tentacles. This homology has been suggested by 

 Van der Hoeven. 



Between the bases of each two lamellaB is a pit lined by 

 exceedingly slender epithelial cells. These cells are also from 

 two to three times the height of ordinary epithelial cells. The 

 cells bear cilia, apparently each cell bearing a single cilium ; 

 but the preservation of my material is not good enougli for 

 me to make sure of this point. Fine fibres appear to run 

 from the bases of the cells into the tissues. 



As has been said, Van der Hoeven's organ occupies the 

 same place in the male that the inferior labial lobe occupies 

 in the female. This organ is about 1 inch in length, f inch 

 in breadth, and § inch in thickness. It is enveloped by a 

 tunic, which, over the anterior dorsal half, is free from the 

 organ, thus allowing free communication between the interior 

 of the organ and the exterior. The anterior half of the organ 

 is separated into halves by a narrow vertical slit which leads 

 into the central cavity. This is a low horizontal cavity ex- 

 tending from side to side. 



At the anterior end, on each side of the opening, are a 

 number of low, thick, vertical lamellae, which quickly pass 

 into thin, broad, shelf-like, horizontal lamellae, which extend 

 as far back as the posterior limit of the vertical slit-like 

 opening of the organ. Back of this point the organ is 

 glandular ; the glands completely surround the central 

 cavity. The glands are typical examples of the compound- 



