176 ZOOLOGY 



[A whole animal stained and mounted may be studied profitably in comparison 

 with the preceding.] 



4. Histology from Sections. — By comparison of longitudinal and transverse 

 sections verify your observations concerning the extent of ectoderm and ento- 

 derm. 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 Muller'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. 



218. For comparison with Hydra secure some alcoholic material of some of the 

 marine hydroids, as Pennaria, Obelia or Campanularia. A tew slides should be se- 

 cured bearing, whole mounts and sections properly stained. 



The following points should be studied briefly: Relation between individuals 

 in the colonies, — branching. What classes of individuals are discoverable, «. e., 

 how do the different branches end? Is there any covering to the softer portions? 

 Tentacles; are they present? If so, what is their arrangement? Hypostome? 

 Mouth? Is there a gastro- vascular cavity? Ectoderm? Entoderm? Give 

 attention to polymorphism among the polyps or zooids. 



219. Metridium (Sea-anemone). — Using well-hardened and preserved speci- 

 mens of sea-anemone make 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 fast- 

 ened 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. Pour or 

 five specimens could thus be used from year to year for demonstration. 



The following studies should be made. Make drawings to illustrate all points 

 observed. 



1. General Form. 



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

 Column. 



Oral disc: zone of tentacles; intermediate zone; lip-zone; mouth; siphono- 

 glyphs (grooves in the angles of the mouth), — number? 



2. Transverse Sections. 

 Body wall. 



Esophagus; does it appear in all the sections? Siphonoglyphs? 



Mesenteries, How is the esophagus held in position? What differences 

 do you find in the mesenteries? They are described as complete (or 

 primary), and incomplete (or secondary, tertiary, etc.). 



