414 HISTOLOGY 



Another organ of attachment is to be found in the cephalopod mol- 

 lusks. These are the ''suckers" which are found on the arms of the 

 various species of cuttles and octopus. They are outgrowths of the arm 

 on its inner side, and each one consists of a cup-shaped organ of muscle 

 and connective tissue, covered with an epithelium which is here simple, 

 and attached to the arm by a strong, muscular stalk of pedicle (Fig. 



375)- 



There are three regions of surface, each covered with a different kind 

 of epithelium on the sucker-cup. The outside is covered with a simple 

 form that is very thin on the proximal part of the outside, and which 

 becomes thicker farther down on the outside, until it is thickest at the 

 rim. On the broad edge it becomes very thick, and secretes a heavy, 

 cuticular ring. This secretion is continued on the inner sides of the cup. 



The bottom of the cup is covered with simple columnar cells that 

 secrete mucus. This bottom is movable up and down in the cylindrical 

 cavity of the cup which is kept rigid and open by the thick cuticle on its 

 sides and edge. The muscle fibers which operate the epithelial "plun- 

 ger" can be seen as parallel fibers passing from the outer surface of the 

 bottom across to the inner surfaces of the plunger. A very narrow layer 

 of muscle across the top, with its fibers lying in all diameters, serves by 

 its contractions to compress laterally and thus to elongate the plunger. 



When elongated, the whole lower surface of the cup is applied to 

 any surface, and then the plunger muscle is contracted. This acts to 

 produce a vacuum in the cavity, and to make the sucker adhere strongly. 

 Weak circular muscle fibers are found in the upper part of the cup's 

 sides. The figure is taken a little to one side of the stalk which, therefore, 

 does not appear, with its core-like continuation, in the center of the suc- 

 tion muscle, but is indicated in outline. 



A very remarkable organ of adhesion is placed on the upper side of 

 the head in the fish, Remora. This sucker acts much as the squid's 

 sucker did, but is more complex in structure. It consists of an oval, 

 pad-like area on the top of the head. The integumental edges of this 

 pad are raised to form a rim, and somewhat similar folds run across its 

 least diameter in parallel lines. Figure 376 shows part of a longitudinal 

 section of the organ, showing one of the ridges. All folds, both the outer 

 edge and the transverse folds are covered with the stratified epithelium 

 characteristic of fish integument. This is the thickest on the edges of 

 the pad and on the tops of the posteriorly slanting folds. It is exceedingly 

 thin and weak in the hollows and on the under sides of the folds. It 

 can thus be seen that the organ does not depend upon any adhesive 

 secretion, and an examination of its mode of operation and muscular 

 structure shows that it depends altogether upon suction. 



The bed of the pad is a thick connective tissue in which lies a thin 



