18 HYDKA. 



thickness of the body-wall. It is coloured green 

 or brown according to the species of Hydra 

 examined. 

 0. The mesogloea, or supporting lamella, 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 upas 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 veiy 

 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 : 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 

 mesoglcBa. 



The shape of these large ectoderm cells 

 varies very greatly with elongation and contrac- 

 tion 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 



