THE YOUNG NATURALIST. 



285 



closely coiled but that an opening or hollow 

 is left in the centre, this hollow is named the 

 umbilicus. The opening of the shell from 

 which the animal emerges is termed the 

 aperture, and the edge of the shell surround- 

 ing the aperture is called the lip. Vv "hen the 

 whorls are raised one above another they 

 form what is called the spire ; the hollow or 

 line which marks the junction of the whorls 

 as they are coiled upon each other is named 

 the suture, 



Some shells have a thin horny external 

 covering which serves as a protection to 

 them, this is called the Epidermis. The 

 Operculum has already been mentioned, it is a 

 thin plate either of a horny or calcare >us 

 substance, and is attached to the animal in 

 such a position that when it retreats within 

 the shell, the operculum closes the aperture, 

 in form and size it therefore corresponds with 

 the aperture, and as the shell grows it is 

 enlarged sometimes by successive layers 

 forming concentric rings, and sometimes 

 spirally. 



The Univalves or Cephala are called also 

 Gasteropoda (belly-footed) from the position 

 of what is termed the foot of the anim d, this 

 is a fleshy disc on the underside of the body 

 on which it creeps or glides along by means 

 of a sort of rippling or undulation of its 

 surface. They possess, as has been said, a 

 well defined head, with two or four tentacles 

 or horns, which can be withdrawn or ex- 

 tended at pleasure; they have eyes placed 

 sometimes at the points of the tentacles, 

 sometimes at their base; the mouth is fur- 

 nished with a number of minute teeth, the 

 viscera and organs of respiration are within 

 the shell. 



Following the classification adopted by 

 Lowell Reeve in his " British band and 

 Fresh-water Mollusks,'* the Cephala, or 

 Univalves, are divided into two tribes, the 

 distinguishing feature being the absence or 

 possession of an operculum, and named 

 therefore Inoperculata, without an operculum, 

 and Operculata, with in operculum. Each of 



these tribes are again subdivided into two 

 separate orders, determined by the mode of 



! respiration, as fitting them to live on land or 



I in water. 



The Inoperculata are divided into the two 

 < orders of Pulmonifera and PulmoLranchiata, the 

 former live altogether on the land, and have 

 a lung adapted for the respiration of air only, 

 the latter possess a compound of lung and 

 gill, enabling them to breathe either air or 

 water, thus living in water as their natural 

 element, the}- are able to exist should the 

 l pond or stream in which they have lived be 

 ' dried up. 



The Operculata are divided into the orders 

 , of Pulmonifera and Braiichifera, the former 

 breathing air only, being furnished with 

 j lungs ; the latter breathing water only, being 

 J furnished with gills. 



| We have now got so far that if a living 

 univalve be taken, (here will be no difficulty 

 in determining to what tribe and order it 

 belongs, it being only necessary to know 

 whether it has an operculum or not, and 



j whether it lives on land or in water. 



The next division is into families. The 

 Inoperculata comprise four families, three 



i belonging to the Pulntonifera, or air-breathing 

 order, namely Limacinea, Colimacea, and Auri- 



| culac .i, and. one to the Puhnobranchiata, breath- 



: ing both air and water, but dwelling chiefly 



j in water, namely Lymnceacea. 



Limaciliea. — The first of these families 

 is that which contains the shell-less mollusks, 



I or slugs, the shell is -ither wanting altogether, 



I or is only rudimentary, it comprises four 



I genera, namely Avion, Geomalacus, himax and 



I Testacella. 



Colimacea. — These possess a spiral shell 

 I in which the alimentary and breathing organs 

 are coiled, they may be distinguished from 

 the following family by the position of the 

 eyes, which are at the points of the tenta-cles. 

 , This is much the largest family, it contains 

 twelve genera and sixty species, or about 

 one-half of the whole number of land and 

 fresh-water shells' the genera are Vitrina. 



