v PRACTICAL DIRECTIONS 325 



6 8 again, noting the various cells and their nuclei, &c. 

 Observe especially 



13. (a) The contractile processes of the ectoderm-cells, which 

 will be cut across transversely, so as to appear as dots just 

 outside the supporting lamella ; (b) the amceboid and 

 vacuolated character of the endoderm-cells. (Special methods 

 of preparation are necessary in order to show the flagella.) 

 Sketch. 



Obelia. 1 



If possible, examine first alive, and then kill and stain as 

 directed in the case of Hydra. 



1. Examine under the low power and note : 



(a) The polypes, with their tentacles and hypostome, 

 expanded and contracted ; and the immature polypes. 



(b) The blasiostyles and medusa-buds, (c) The ccenosarc and 

 the perisarc, hydrothecce, and gonothecce (Fig. 78). Sketch. 



2. Then stain, put on the high power and make out the 

 minute structure of the polypes, noting the 



(a) Mouth, Cb) enter on, (c) ectoderm, supporting lamella, 

 and endoderm (solid in the tentacles). On the blastostyles 

 examine the medusa-buds. (If you wish to make permanent 

 preparations, mounted in balsam, use the method given for 

 Hydra.) 



Sketch an optical section of a polype and blastostyle. 



Sections of an entire branch, prepared in the usual way 

 (see p. 136), should also be made. Select for examination 

 those which pass as nearly as possible through the vertical 

 and transverse axes of a polype, and compare with your 

 sections of Hydra. 



3. Place a medusa (Figs. 78 B D and 79) on a slide with 

 the sub-umbrella surface uppermost, stain, and mount 

 carefully in glycerine. Note 



(a) The umbrella, (b) manubrium and mouth, (c) tentacles, 



(c) radial and circular canals, (d) velum, (e) gonads, and 

 (/) lithocysts (often difficult to recognise in preserved speci- 

 mens). Sketch. 



1 Specimens living or preserved, both of the colonial and 

 medusa-stage of Obelia or some other Hydroid (as well as other 

 marine animals described in this book) can be obtained from any 

 Marine Biological Laboratory ; or the fresh-water Cordylophora 

 will answer the purpose as far as the colony is concerned, but it 

 has no medusa-stage. 



