ADHESION AND SPINNING 



413 



a shallow gland as a calcified shell-like structure which is fastened per- 

 manently to the rock. 



The sucking disk of the fish-leech, Pontobdella, while used as a me- 

 chanical sucker, is also to some extent an adhesive .instrument. The 

 epithelium that covers the applied surface of this organ is stratified, and 

 the small amount of adhesive material is more of a mucus than a glue. 



ped ) 



mus 



s.ep. 



FlG- 375. Axial section through the grasping disk on the arm of a cephalopod mollusk; the 

 squid, Loligo Pealii. s.ep., simple epithelium becoming thickened toward rim; sec.ep., se- 

 cretory portion of simple epithelium, found on inner sides of cup to secrete the stiff cylinder 

 (cy.) which is here distorted in the process of preparation; p.ep., epithelium which acts as 

 a portion or plunger to produce a vacuum; mus., muscle which operates the piston; ped., 

 muscular pedicel (in outline) by which the organ is attached to the arm. X 600. 



Several species of Aphrodite, another annelid, produce very large 

 quantities of an adhesive material which is used to build the tubes in 

 which the animal lives. This material is secreted by epithelial cells 

 which are invaginated into glands. These glands correspond to the 

 upper setigerous glands which, in other worms, are used to produce the 

 solid setae. 



