v PRACTICAL DIRECTIONS 325 



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 amoeboid 2a\& vacuolated character of the endoderm-co\\.s. 

 (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 blastostyles and me- 

 dusa-buds, (c} The ccenosarc and the perisarc, hydrothecce , and gono- 

 thectz (Fig. 78). Sketch. 



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

 structure of the polypes, noting the 



(a) Mouth, (b} enteron, (c) ectoderm, supporting lamella, and endo- 

 derm (solid in the tentacles). In the blasto-styles 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. 324) 

 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} manubriuni and mouth, (c) tentacles, (c] radial 

 and circular canals, (d} velum, (e) gonads, and (f) lithocysts (often 

 difficult to recognise in preserved specimens). 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. 



