22 FtlESH-WATER AQUARIA. 



life, these l3ein« their natural food. There sliould geuerally be a fair 

 number of snails present, as they greatly assist in keeping the vegeta- 

 tion within reasonable bounds, and being particularly fond of confervjB, 

 and very ravenous, the glass is usually kept fairly free fi'om it, which, 

 as we have seen, is desirable. The best species are Lymnaa stagnalis, 

 Pahulina vivipara, P. contecta, and any of the larger species of 

 Planorbis ; Plaiiorbis corneus being the largest, is to be preferred, but 

 P. carinatus, P. spirorbin, as also Lijmnaa pcrcfim and Bi/thinia 

 tentaciilata are by no means unsightly, and are only inferior in point of 

 size to the others above mentioned. Snails are exceedingly inter- 

 esting objects in the very young state under the microscope, and the 

 process of development in the egg can be better studied in those 

 of the fresh-water moUusca than perhaps in any other. Bythinia 

 tentaciilata is the one of all others with which I am acquainted that 

 arranges its eggs in the most convenient form for observation. They 

 are placed in rows of two or three abreast and never on the top of one 

 another. They are, moreover, of a comparatively large size, and their 

 envelopes are very pellucid. The young of the Lymutea, as also the 

 full-grown Phys£E, are very interesting to watch as they ascend and 

 descend through the water by means of a mucus thread which they 

 secrete, but which, ordinarily invisible, can be shown to be present by 

 passing a solid body such as a glass rod between the animal and the 

 point of attachment of the thread. The large bivalves, Unio and 

 Anodouta, may be introduced into the aquarium, and they assist in keep- 

 ing the water clear, but they draw into their systems through their 

 fringed syphon tubes diatoms, desmids, rotifers, and other small swim- 

 ming and floating organisms. If they are in a tank they should be 

 watched and removed as soon as death occurs, which may be known by 

 the gaping of the valves of the shell, since such a large mass of decom- 

 posing animal matter kills some of the other inhabitants very quickly, 

 besides giving off a hy no means agreeable smell. They will, however, 

 under ordinary circumstances, live several years in confinement. 



Polyzoa are geuerally not difficult to keep in an ordinary aquarium 

 in moderate quantity. The statoblasts or " winter eggs" should be 

 allowed to remain in the water, for though some of them float on the 

 top of the water, or cling to the sides of the tank, giving it a somewhat 

 untidy appearance if they are in considerable numbers, yet the beauty 

 they display when they burst, and the young individuals come forth, 

 amply atones for the former slight unsightliness. The statoblasts 

 may, however, if desired, be removed, and placed in an auxiliary tank 

 to be kept through the winter. Indeed, it is a verv good plan to keep 

 specially interesting microscopic beings in separate small glass vessels, 

 as they are the more conveniently got at when required for examination, 

 and can also the better be preserved from their natural enemies.* 



* Mr. Potts has introduced au admirable form for this and other purposes. It 

 is in shajae a large Zoophyte trough about 4in. iu height aud width. It has au 

 extended base, so that it will stand in a window, and the depth from front to 

 back being liij., a pocket lens can conveniently be brought to bear upon any- 

 thing in it. They are to be bad at Miss Bailey's, in Bennett's Hill, 

 Birmingham. 



