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 mesoderm, or supporting lamella, is a very 



thin homogeneous layer between the ectoderm 

 and 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 layers 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 mesoderm, are narrower, and separated 

 from one another by intervals of varying size. 

 The shape of these large ectoderm cells 

 varies very greatly with elongation and con- 

 traction of the tentacle. When contracted 

 they are transversely elongated, and their 

 outlines are very distinct ; when expanded 

 their length considerably 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. 



iii. The nematocysts or ' thread cells ' are sharply 

 defined oval capsules filled with fluid and en- 



c 



