SUGGESTIONS FOR THE LABORATORY ASSISTANTS 143 



last-named material. We have found that a dilute solution of formalde- 

 hyde is remarkably effective for the purpose. Add one drop of formal- 

 dehyde to 100 c.c. of water, and test it on Paramecium. If too strong 

 continue to dilute until a strength is found which seems to have no effect 

 on the animals immediately but in a few minutes causes them to slow 

 down very gradually. The solution may be used as directed in the manual 

 or a drop of solution may be placed alongside the edge of the cover glass. In 

 the latter case a stronger solution will be required. If the assistant will take 

 the trouble to find out the proper strength by trial in advance, he will find the 

 method to be very successful. 



12. Section on Hydra. To obtain Hydra collect aquatic plants, particularly 

 soft ones, from clear ponds or bays in sluggish streams. Place the plants in 

 jars, using a large amount of water to a small quantity of the plants. As the 

 animals become noticeable, they may be picked out and placed in a smaller 

 vessel. If they are to be kept for any length of time they must be fed with 

 Entomostraca. To raise Entomostraca place two or three inches of pond mud 

 in a glass vessel, fill with water, and throw in from time to time a few grains of 

 boiled wheat. If Entomostraca do not appear of themselves in such a culture, 

 as they usually will, a few to start it should be obtained from ponds. It is 

 possible by this method to grow the Hydra and the Entomostraca in the same 

 vessel, by keeping the fermentation of the wheat to the lowest possible level. 

 The work with the structure of the living animal is always rather difficult for 

 students and may profitably be omitted. Small individuals are best for the 

 purpose. 



13. Section on Planaria. Planaria occurs in spring-fed pools and in ponds. 

 To collect from springs, hang a piece of meat in the current for an hour or two, 

 whereupon Planaria, if present, will attach to the meat and may then be shaken 

 off into a bottle. Collect quantities of plants from ponds and place in pans 

 with a small quantity of water. After a few days, as the plants decay, Planaria, 

 if present, will gather at the surface and should be removed. Planarians are 

 very intolerant of stagnant water and must be kept in large open pans in which 

 the water is frequently changed. They should be fed every few days with fresh 

 liver. The liver must always be removed within a few hours and the pan and 

 worms thoroughly washed. The digestive tract of Planaria will always stand 

 out plainly if the anaesthetized animals are pressed out as described in the 

 manual. However, it can be made to stand out beautifully on the intact animals 

 by feeding them on blood clots shortly before they are to be used. To demon- 

 strate the feeding of Planaria use worms which have been starved for a week 

 or two. 



14. Section on the earthworm. The dissection of the reproductive system 

 should ordinarily be omitted. Directions are given merely for the sake of com- 

 pleteness. 



