cilia . The xiilia maintain the surfaces free of sediment, understandably a major 

 problem with gastropods which live on the bottom in water that contains varying 

 quantities of sediment in suspension. 



The internal surfaces of the shell are blanketed by a sheet of tissue, the 

 mantle, which becomes thickened near the shell aperture for shell secretion as 

 the animal grows . Over the back of the drill and under the dorsal anterior portion 

 of the shell and mantle there lies a large conspicuous chamber, the mantle cavity, 

 which shelters a number of vital organs and openings. On the left side the mantle 

 projects as a specialized tube into the siphonal canal of the shell. All inner 

 surfaces of this tube and mantle cavity bear cilia which propel water by Tvay of 

 this tube into the mantle cavity and eject it from the right side of the mantle 

 cavity. As water enters this chamber it first strikes a specialized sensory surface, 

 the osphradium. This, according to the convincing studies of Yonge (1947), estimates 

 the quantity of sediment carried in the water. Such a function appears of extreme 

 importance to the drill which lives directly on the substratum where the danger of 

 fouling or blocking of the mantle cavity is a constant one . It is supposed that in 

 heavy suspensions of sediment the drill closes within its shell, although the re- 

 actions of Urosalpinx under these circumstances have not been described. That 

 some drills do survive in densely turbid water has been demonstrated in portions 

 of Delaware Bay where dense populations of Urosalpinx live in water so roiled much 

 of the time that a Secchi disc disappears at 0.1 to 0.2 meter (T.C. Nelson, pers.com.), 



Immediately adjacent to the osphradium and extending the full length of the 

 mantle cavity lies a large double -comb shaped gill. Probably much of external 

 respiration is accomplished during the passage of sea water over this organ. Out 

 of their native medium drills remain alive only so long as the mantle cavity is 

 moist; by crawling about they accelerate the loss of water and hasten their de- 

 struction . 



The anus in both sexes and the vagina in the female also open into the 

 mantle cavity on the right side at the point where water is pumped to the outside . 

 Another useful organ to the drill, a very large mucus gland, covers much of the 

 dorsal half of the mantle cavity. It secretes copious quantities of a sticky fluid 

 which entangles irritating sand and silt particles (Yonge, 1947) which in turn are 

 removed from the mantle cavity by water currents on ciliary pathways . The 

 presence within the mantle cavity of a well protected gill, an effective sediment 

 testing organ in company with a highly efficient self cleansing mechanism, helps 

 to explain the high degree of adaptability of this muricid to a wide range of 

 habitats . 



19 



