STRUCTURE OF TENTACLE 17 



b. The endoderm is the inner layer, lining the diges- 



tive cavity, and forming about two-thirds of the 

 thickness of the body-wall. It is coloured green 

 or brown according to the species of Hydra 

 examined. 



c. The supporting lamella, or mesogloea, is a very 



thin gelatinous layer between the ectoderm and 

 the endoderm, and hardly visible with the low 

 power. 



2. Structure of the tentacles. 



Examine one of the tentacles with a high power, and note 

 its division into layers corresponding to those of the body-wall. 

 Examine (1) the middle of the width of the tentacle, altering 

 the focus so as to bring the several la/yers into view in turn; 

 and, (2) the edge of the tentacle, ivhere by proper adjustment 

 of the focus a profile view of the cells can be obtained. 



a. The ectoderm is a cellular layer made up as follows, 

 i. The large ectoderm cells form a single layer 

 covering the whole surface of both the tentacles 

 and the body. They are somewhat conical 

 in shape. , Their outer ends are closely fitted 

 together, forming a mosaic pattern very 

 clearly visible in surface views of the ten- 

 tacle. Their inner ends, which rest directly 

 on the supporting lamella, are narrower, and 

 separated from one another by intervals of 

 varying extent. 



The shape of these largef ectoderm cells 

 varies very greatly with elongation and contrac- 

 tion of the tentacle. When contracted they are 

 widened transversely, and their outhnes are 

 very distinct ; when expanded their length con- 

 siderably exceeds their width. 

 ii. The interstitial cells are small rounded cells 

 placed in little clusters between the inner or 

 basal ends of the large ectoderm cells. 



c 



