'^' PRACTICAL DIRECTIONS 315 



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Note the brnwn colour in //. fusca, and the green colour in //. viridis. 

 Observe the method of seizing food. Place a specimen on a slide in a 

 drop of water, together with a small piece of water-weed or paper to 

 prevent crushing, and then put on a cover-glass. Wait till the animal 

 is fully expanded, and then examine with the low power. Note : — 



1. The body, enclosing the digestive cavity or enteron, which opefts 

 by the mouth on the free or distal end of the animal, at the summit of a 

 conical hypostome. At the proximal end is the flattened ^(;/ or disc of 

 attachment. 



2. The tentacles, arranged in a single circlet or whori. around the 

 base of the hypostomc. They are hollow, and their cavities communi- 

 cate proximally with the general digestive cavity of the body. On their 

 surface are a number of small knobs. 



3. The great contractility of the animal, and especially of the ten- 

 tacles. 



4. The structure of the body-wall, which is made up of (a) an outer 

 layer of colourless cells {ectoderm); and {b) an inner layer (brown in 

 H. fusca and green in H. viridis) of cells {endoderm) lining the diges- 

 tive cavity. Between these two layers is » thin gelatinous non-cellular 

 supporting lamella or mesoglcea, not easily seen with the low power. 

 (The tentacles have a similar structure, the details of which cannot be 

 made out with the low power. ) Sketch. 



Put on the high power and examine a tentacle, focussing on to the 

 surface as well as deeper, so as to get an optical section. Note ; — 



5. The relations of the ectoderm, endoderm, and supporting lamella, 

 and the nuclei of the ectoderm and endoderm cells. 



6. The structure of the ectoderm : — (a) large conical cells, with their 

 broader ends outwards, arranged in a single row, and dififering in form 

 according to the state of contraction. The spaces between the inner 

 narrower ends of these are filled up with {b) smaller, rounded interstitial 

 cells (absent on the foot) ; (c] thread- cells or nematocysts (Fig. 7S) — oval 

 capsules containing a spirally-wound thread, developed within certain 

 of the interstitial cells called cnidoblasts, and when fully formed, found 

 imbedded in or between the large ectoderm-cells. They are much more 

 numerous on the tentacles than on the body, causing the knobs referred 

 to above. Each cnidoblast gives rise to a small process — the trigger- 

 hair or cnidocil, which projects from the surface. Notice the discharged 

 thread-cells, and observe that each consists of a flask-like base (to which 

 part of the protoplasm and the nucleus of the burst cnidoblast usually 



