APPENDIX. 411 



of sponges is difficult for the beginner. The arrangement of 

 the canals and the microscopic structure of the skeleton of the 

 Toilet Sponge should be studied (Fig. 190). Specimens for 

 this work may be purchased at any drug-store. Then alcoholic 

 specimens of similar sponges, in which the flesh has been pre- 

 served, may be sectioned in various directions with a sharp 

 knife, and the difEerence between the skeleton and the complete 

 body noted. Sections of Grantia, or of some other simple 

 sponge prepared in such a way as to show the canals and ciliat- 

 ed chambers, as well as the young in various stages of develop- 

 ment, may be purchased from any dealer in microscopic supplies 

 (Fig. 189). 



CcELENTE RATES. — If the fresh-water Hydra (Fig. 191) can be 

 obtained it will be found useful to illustrate the structure of 

 the Ccelenterata. It lives in clear water in ponds and ditches, 

 and attaches itself to the stems and roots of floating plants, as 

 duck-weed, various algse, etc. Either the green or the brown 

 form may be used. The animal may first be examined in the 

 aquarium, and the movements of its body and tentacles noted ; 

 also its method of locomotion. Then it may be placed in a 

 watch-glass, and studied under the low power of the micro- 

 scope ; small scraps of fresh meat not so large as a pin-head 

 may be given it, and its method of feeding watched. If placed 

 on a slide in a drop of water and covered it may be examined 

 with a higher power and the structure noted — the body-wall 

 consisting of cells arranged in two well-defined layers, many of 

 the cells containing green particles ; the digestive cavity ex- 

 tending throughout the body and into the tentacles ; from some 

 of the cells the nettling - threads may be seen to shoot out. 

 Look for specimens bearing buds. Prepared slides showing 

 longitudinal and cross sections of the body may be purchased. 



If hydras cannot be obtained, some of the marine hjdroids, 

 either living, alcoholic, or stained specimens mounted on slides 

 should be studied. The campanularian, hydroids are suitable. 

 These are colonial forms, and in each colony both feeding and 

 reproductive zooids will be found, as well as young zooids in 

 various stages of development from the first formation of the 

 bud to the full-grown zooid. Study live specimens in sea-water, 

 noting particularly their movements, and, if possible, their meth- 

 od of feeding. Examine alcoholic specimens in a watch-glass 

 containing fifty per cent, alcohol. Note the plant-like aspect 



