22 



small teeth. Several rows of conical teeth, the accessory teeth, which gradually 

 increase in heiglit from williin outward, arc borne h\' tlic edge of the sucker 

 and reinforce the teeth of the i-ing. Kach accessory tooth is secreted by a 

 single large epithelial cell. The ring and accessory teeth are composed of a 

 chitin-like substance which differs from chitin in being soluble in dilute alkalies. 

 The external surface of the sucker is covered by low cubical or flat epithelium 

 and the sides of the cup are covered by a thicker epithelium which forms short 

 tubular glands. The inner surface of the side of the cup is covered by a 

 columnar epithelium which secretes the ring and whose height at each point 

 is proportional to the thickness of the corresponding ]3ortion of the ring. The 



i-=-- 



B 



Fis. 9. 



base of the cup is lined by a columnar epithelium , whose cells are several 

 times as high as broad. The po.sition of the nuclei of these cells varies greatly , 

 in some cells the nuclei are at the base, in others at the middle, and in yet 

 others at the free end of the cells. 



The unsymmetrical suckers (Plate I. Fig. 4) have the basal muscle, the 

 sphincter, and the lower ring muscle excessively developed on the large side 

 and correspondingly reduced or lacking on the small side. The smaller side of 

 the ring (text figure 9) is strongly ridged and is toothless while the large side 

 lacks the ridge and has from five to seven large , strong , chisel-shaped , and 

 incurved teeth. These suckers can serve only as hooks but the symmetrical 

 suckers may be pressed against the surface of objects until the water is driven 



