v PRACTICAL DIRECTIONS 323 



by the mouth at the distal end of the animal, at the summit of a conical 

 hypos tome. At the proximal end is the/w/, or disc of attachment. 



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

 base of the hypostome. 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 contractions of the animal, and of its tentacles. 



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 a thin gelatinus 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 (Fig. 76, A). 

 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 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 nematocysts (Fig. 77) 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 

 remains attached) and a long filament, with three large and several 

 smaller spines or barbs at its proximal end. (Smaller thread-cells, 

 with thicker threads and no spines, are also present ; some of these 

 have long, spirally coiled threads, others shorter, straight threads. These 

 can be seen better later on. ) 



7. The endoderm, consisting of a single layer of large amoeboid cells, 

 which in H. viridis contain green Zoochlorellce (p. 304). Note the 



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