PRACTICAL DIRECTIONS 323 



summit of a conical hypostome. At the proximal end is the 

 foot, 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 communicate proximally with the general 

 digestive cavity of the body. On their surface are a number 

 of small knobs. 



3. The contractions of the body and 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 digestive cavity. Between these 

 two layers is a 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 (see p. 558 and Fig. 76, A). Note : 



5. The relations of the ectoderm, endoderm, and support- 

 ing 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 later on.) 



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

 amoeboid cells, which in H. viridis contain green Zoo- 



Y 2 



