CCELENTERATA. 167 



in the nettle cells? [A whole animal stained and mounted may be studied 

 profitably in comparison with the preceding.] 



4. Histology from Sections. If the teacher is not equipped for im- 

 bedding and sectioning objects, and desires to carry this work further, 

 stained and mounted sections of Hydras and most of the other prepared 

 sections suggested in this book can be secured for a reasonable sum by 

 applying to any of the large laboratories. By comparison of longitudinal 

 and transverse sections verify your observations concerning the extent of 

 ectoderm and entoderm. What occurs between the layers? Study the 

 shape and arrangement of the cells in both layers. Compare as to size. 

 What is the relation of the nettle cells to the other ectodermal cells? 



5. Histology from Maceration Preparations. Place a specimen in a 

 watch glass, and draw away some of the water with a pipette. When the 

 Hydra is well extended, pour over it an aqueous solution of hot corrosive 

 sublimate. Rinse and place in Miiller's fluid or 15% alcohol for 24 hours. 

 Take a portion of the body and place on a slide in a drop of glycerine 

 and water. Cover, and tap the cover-glass very gently with a needle. 

 The cells thus become separated, and their shape may more readily be 

 seen. Instructions for staining may be found in texts on histology. 



Study the nettle cells, the ectodermal cells, the entoderm, and the 

 gland cells of the foot and gullet. 



213. For comparison with Hydra the teacher should secure some alco- 

 holic material of some of the marine hydroids, as Pennaria, Obelia or 

 Campanularia. A few slides should be secured bearing whole mounts and 

 sections properly stained. 



The following points should be studied briefly: Relation between indi- 

 viduals in the colonies, branching. W 7 hat classes of individuals are dis- 

 coverable, i. e., how do the different branches end? Is there any cover- 

 ing to the softer portions? Tentacles; are they present? If so, what is 

 their arrangement? Hypostome? Mouth? Is there a gastro-vascular 

 cavity? Ectoderm? Entoderm? Call attention to polymorphism among 

 the polyps or sooids. 



214. Metridium (Sea-anemone). If lack of appropriations will not 

 allow the purchase of sufficient material for class work, the teacher should 

 have at least a few well hardened and preserved specimens of sea-ane- 

 mone. From these should be made a series of cross-sections from various 

 parts of the body, with a thickness of one-eighth to one-fourth inch. 

 These sections may be fastened to cards or to plates of mica by thread 

 or fine wire and kept in preserving fluid. One specimen^should be split 

 lengthwise, and one left whole. Four or five specimens could thus be 

 used from year to year until more abundant supplies are obtained. 



The following studies should be made. Make drawings to illustrate 

 all points made out. 

 i. General Form. 



Base, or aboral disc (the end attached during life). 

 Column. 



