IV PRACTICAL DIRECTIONS 315 



Xole till.' brown colour in // fiisca, and the green colo\n' in //. vin'n'is. 

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

 drop ol water, together with a small piece r)f 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 eavity or enteron, which opens 

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

 conical hyposlouie. \\. the proximal end is the flattened/^')'/ or disc of 

 attachment. 



2. The leniailes^ arranged in a single circlet or w/iori around the 

 base of the hypostome. They are hollow, and their cavities comnumi- 

 cate proximally with llie general digestive cavity of the bod;-. On their 

 surface are a number of small knobs. 



3. The great contractilit\' of the animal, and especial!)" of the ten- 

 tacles. 



4. The stnictiire of iJie hody-7uaJl, which is made up of (rt) an outer 

 layer of colourless cells [ectoderm] ; and (/') an inner layer (brown in 

 H. fusca 2cni green in j^. viridis) o{ c&Wi (endodcrin) lining the diges- 

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

 supporting /aniclla or tiiesogleea, 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, eiidoderm, 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 differing 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 ncmatocysts (Fig. 75)— oval 

 capsules containing a spirally-\s ound thread, developed within certain 

 of the interstitial cells called c;/;'a'(7WflJ-/.v, and wdien fully formed, fiamd 

 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 nucletis of the burst cnidoblast usually 



