130 Invertebrate zoology. 



different parts. The cerebral ganglia lie above the inoiith, upon the anterior 

 adductor mnscle. The visceral ganglia lie upon the posterior adductor, under- 

 neath. The pedal ganglia lie in the anterior part of the foot. The commis- 

 sures are the cerebro-visceral, lying superficially above the base of the gills ; 

 and the cerehro-pedal, through the visceral mass. 



Type XX.— Helix Pomatia. 



(This is the large vineyard snail of Europe. Any large native Helix maybe 

 used instead.) 



I 02. Ohtaining and preserving. Helix Pomatia is used in Europe as a 

 common article of diet. It hibernates under leaves and stones, first closing 

 the month of its shell vdth a layer of lime {epiphragma). In this state it is 

 gathered in quantity and sent to the cities, and may be occasionally obtained 

 in the large restaurants of American cities (Philadelphia). When obtained, 

 break out the epiphragma and the snail will resume its activity. The com- 

 mon New^ England form. H. albolabris, is very typical in structure, but much 

 smaller and seldom found in quantity sufficient for class use. It occurs in 

 damp woods, crawling over moss or resting beneath stones or pieces of de- 

 cayed wood. The ordinary methods of killing cannot be applied to snails, as 

 they will withdraw into their shells at the least alarm. To obviate this, use 

 the following method ; — Place the snails in a jar which can be hermetically 

 closed, and fill it entirely with water, by immersing the whole in a pail, and 

 closing the jar while immersed. In this the snails will become slowly asphyx- 

 iated (24-36 hours) and will remain exxmnded. after which they can be fixed 

 and preserved in the usual way. Water from which the air has been expelled 

 by boiling will accomplish the end more speedily. The result may be also 

 hastened by the addition of a little chloral hydrate (2-3 fO- 



I 03. External characteristics. An expanded animal may be seen to con- 

 sist of three portions ; head, foot and visceral sac. The latter is contained in 

 a sjiiral shell, which may also contain the other two when contracted. In 

 what respects does the shell differ f rrau that of a clam ? Is it bilateral ? If 

 it were plastic and capable of being unrolled, could it be placed so as to be- 

 come so ? What would then be the shape of the visceral sac V Are the head 

 and foot bilateral ? Is it probable that the shell and visceral sac were origi- 

 nally bilateral also ? Is the shell carried exactly on the top of the animal, or 

 inclined to one side ? Is this always the same side? Which? Viewing the 

 shell from above, it is seen to consist of a gradually expanding spiral, wound 



