472 ZOOLOGY. 



ill. or so in diameter, in its liver and mesenteries ; these are 

 encysted tapeworms (Tsenia), unable to continue their development 

 until introduced into the stomach of a carnivorous animal. The 

 frog may have its lungs full of black and white worms (Ascaris), and 

 one or two six-suckered worms (Polystomum) in its urinary bladder. 

 Long white threadworms infest all parts of the dogfish. A 

 1'arameci um-like Protozoon is generally found running over the 

 surface of Hydra. The earthworm, thanks to its mode of life, is 

 specially given to unintended hospitality : its nephridia are never 

 free from minute wriggling threadworms, and the fluid in its seminal 

 vesicles contains spore-forming Protozoa, mistaken by the unwary 

 for sperm ospores: other stages of the same Protozoon may be seen 

 attached to the funnels of the vasa def erentia. The gills of Anodonta 

 harbour a little spider-like creature which the uninitiated mistake 

 for a glochidium. On the hind-gut of Astacus, egg-shaped orange 

 bodies may be found (encysted Echmorhynchus). 



For the re-agents used in staining and mounting animal 

 tissues we must refer the student to the works before 

 mentioned. For the rough examination of fresh tissues, 

 normal salt solution (| per cent., or 7*5 grammes common 

 salt to a litre of water) is used. If the object appears too 

 opaque in this fluid, it can be mounted in weak glycerine 

 (not less than half water). In all cases a cover-glass should 

 be carefully placed over the specimen, and none of the 

 liquid should be allowed to get on the upper surface of the 

 cover-glass : if any does, don't try to wipe it off remove the 

 cover-glass and put another on. 



We cannot here give instructions in the way of using a 

 microscope : printed instructions, we know from experience, 

 are of practically no value. More can be learned in one 

 hour's lesson from a practical teacher than from many pages 

 of a book. Failing such direct instruction, we must again 

 refer the student to the practical books we have recom- 

 mended. 



Heady prepared histological sections, as well as general 

 sections of such animals as Amphioxus, Lumbricus, and 

 Hydra, can be obtained from the dealers already mentioned 

 and other dealers in microscope-slides. 



EMBRYOLOGY. 



The student should look out for frog-spawn on ponds in 

 March, and keep some in a glass jar with a little pond-weed 



