4io 



HISTOLOGY 



ad.sec. 



FIG. 370. Three epidermal cells 

 from the proximal surface of the 

 " foot " of Hydra, b.m., basement 

 membrane; ad. sec., granules of 

 adhesive secretion on the distal 

 parts of the threads. X 870. 



sec.g 



specialization that is necessary in so 

 small animals. The attachment is either 

 temporary or permanent. 



The Coelenterates are rich in the num- 

 ber of forms that attach themselves by 

 some part of their surface. Hydra will 

 show an evident example of a typical 

 form of adhesive structure. Sections of 

 the epithelium on the "foot" show that 

 this tissue has become thicker and stronger 

 by the development of this power (Fig. 

 370). On closer examination one can see that the individual cells have 

 well developed fibrils in their cytoplasm reaching from proximal to 

 distal surface and better developed distally than 

 proximally. This is a contrast with the few 

 weak fibrils found in other epithelial cells on 

 the body because these latter are far more ir- 

 regular in direction and thus not so well fitted 

 mechanically to bear a strain. The next point 

 to be noticed is that in the outer fifth of the 

 cytoplasmic body of the cell, a number of se- 

 cretion granules appear lying against the fibrils. 

 These are undoubtedly secretion granules of the 

 adhesive material that is used to make the foot 

 stick to the surface on which the animal rests. 



Other surfaces on the various Cnidaria have a similar structure, some- 

 times much specialized, but developed on the same principle. We shall 



go to the Ctenophora and ex- 

 amine a specimen of epithelium 

 that is used, not to hold the 

 body in any particular position, 

 but to seize the prey upon 

 which the animal feeds. 



The tentacular epithelium 

 of Beroe ovata has been de- 

 scribed by Schneider, and 

 shows this seizing apparatus 

 splendidly developed (Figs. 371, 

 372, and 373). The apparatus 

 is not unicellular but consists 

 of a number of groups of cells, 

 each group consisting of from 

 three to seven cells extending 



g. c. 



FIG. 371. Very young stage 

 of a group of grasping cells 

 from Beroe ovata. g.c., two 

 young grasping cells ; sec.g., 

 secretion granules ; c.c., 

 single cap cell. (After 

 SCHNEIDER.) 



n.c.t. 



FIG. 372. Half-developed group of grasping cells 

 from Beroe ovata. ., nucleus of grasping cell; 

 n.c.c., nucleus of cap cell; n.c.t., nucleus of con- 

 nective-tissue cell; sec.g., secretion granules 

 (poison?); ad.s., adhesive substance; mu.c., 

 mucin cells. (After SCHNEIDER.) 



