CLASS II. GASTEROPODA: ORDER 1. PULMONIFERA. 505 



Class II. OA^Ti:ilOPOI>A. 



This term, derived from the Greek gaster, the belly, and po?/s, foot, signifies helhj -walkers. 



.__ The organ by which they mo\e, 



as is shown in the common snail, 

 consists of a broad, muscular, 

 disk-like foot, attached to the 

 ventral surface, upon which the 

 animal creeps very slowly with 

 a gliding motion. The muscu- 

 lar movements may be seen fol- 

 lowing each other in rapid waves, 

 when a snail is climbing a pane 

 of glass. There are numerous 

 species, which greatly vary in 

 foi'm. All, however, have a dis- 

 tinct head ; respiration is effected 

 by branchiae, or a pulmonary sac ; 

 the organs of the senses are ten- 

 tacles of various forms ; the eyes 

 are usually placed at the ends of 

 tentacles situated upon the head. 

 No special organs of taste or smell have been detected, but there is good reason, from the discri- 

 mination these animals show in the selection of their food, for believing that they possess them. 

 The general form of the body is characteristic of the class ; from the preponderance of one side 

 of the body, the whole, during growth, acquires a spiral form; it is only in some naked species 

 that we find the body symmetrical. Most of the Gasteropoda close the aperture of their shell 

 by a horny or calcareous plate, called the operculum. Most of the species are oviparous ; a few 

 are ovo-viviparous. The sexes are generally separate, but many are hermaphrodites. The 

 young are always provided with a shell while in the Qgg. These animals are divided into three 

 orders, the Pulmonifera, Branchifera, and Heteropoda. 



THE COMMON SNAIL. 



ORDER 1. PULMONIFERA. 



This term, from the Latin, pulmo^ a lung, and/ero, to bear, refers to the fact that the animals 

 of this order breathe air by means of lungs, and not water by means of branchiae. It includes 

 several species popularly known under the name of Snails and Slugs. 



THE HELICID^. 



Genus HELIX : Helix. — To this belongs the Common Garden-Snail, H. aspera. This is 

 furnished with four tentacula, two of which are smaller than the others ; at the end of these, 

 which the animal pushes out or draws back like telescopes, are blackish knobs, which are the 

 eyes. It lays eggs about the size of peas, which are of a soft transparent substance. By closely 

 examining with a magnifying lens, the young snail may be seen in the egg, with its embryo shell 

 on its back. The snail is extremely tenacious of life, in evidence of which numerous examples 

 have been cited ; among them is the following, which is furnished by Mrs. Loudon : a Mr. S. 

 Simon, a merchant of Dublin, whose father, a Fellow of the Royal Society, and a lover of natural 

 history, left him a small collection of fossils and other curiosities, had, among them, the shells 

 of some snails. About fifteen years after his father's death, he gave to his son, a child of ten 

 years old, some of these snail-shells to play with. The boy placed them in a flower-pot, which 



Vol. II.— 64 



