v 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 (6) an 

 inner layer (brown in H. iusca, and green in H. i iridis) 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 cnidoblasfs, 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 



