22 HYDRA. 



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, 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 breadth of the tentacle,, altering 

 the focus so as to bring the several layers into view in turn ; 

 and (2) the edge of the tentacle, where 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 pouter ends are closely fitted 

 together, forming a mosaic pattern very clearly 

 visible in surface views of the tentacle. Their 

 inner ends, which rest directly on the support- 

 ing lamella, are narrower, and separated from 

 one another by intervals of varying extent : 

 these inner ends are produced into slender 

 muscular processes or ' tails,' which lie parallel 

 to the long axis of the tentacle, and closely 

 applied to, or imbedded in, the mesogloea. 



The shape of these large ectoderm cells 

 varies very greatly with elongation and con- 

 traction of the tentacle. When contracted they 

 are widened transversely, and their outlines are 

 very distinct ; when elongated their length 

 considerably exceeds their width. 



ii . The interstitial cells are small rounded cells placed 



