104 ZOOLOGY. 



spiral are called whorls. The last whorl is the largest, 

 and is termed the body whorl. The part of the shell 

 above the body whorl is the spire. The sutures are the 

 lines of junction of the whorls. Sometimes the whorls do 

 not form a, spire, but are in the same plane, forming a 

 disc, as in the fresh water shell Planorbis. In the vegetable- 

 feeding gasteropods, the aperture or mouth of the shell is 

 unbroken or " entire." These are therefore called holo- 

 stomata (Gr. holos, whole; stoma, mouth). In the carni- 

 vorous species, the mouth of the shell is notched or drawn 

 out into a canal for the protection of a breathing siphon. 

 Sometimes there are two of these canals or notches. 

 These breathing tubes have no immediate connection with 

 the carnivorous habits of the animals. 



144:. Digestion. The head is very distinct, and is 

 usually furnished with two long tentacles. There are also 

 often two stalks, on the summit of which the eyes are 

 placed. The mouth is sometimes furnished with two 

 homy jaws. There is always an odontophore, armed with 

 siliceous teeth, similar to that possessed by the Pteropoda 

 and Cephalopoda. The mouth leads to a gullet, which is 

 succeeded by a stomach and convoluted intestine. In 

 vegetable feeders, the stomach is often provided with 

 calcareous plates, which assist in grinding the food. The 

 anus is placed on the right side, in the front part of the 

 body. There are salivary glands and a liver. 



145. Circulation. There is usually a distinct heart, 

 with two cavities an auricle and a ventricle. 



146. Respiration is variously effected in the different 

 groups. The land snails and slugs, and some fresh water 

 gasteropods (Limncea, &c.), breath air directly by means 

 of a pulmonary chamber hollowed out on the right side 

 of the body near the head. This chamber is provided 

 with an opening to admit the air, and its walls are lined 

 with a membrane containing numerous blood-vessels. 

 This group of air-breathers is placed by Professor Huxley 

 in a separate class the Pulmo-Gasteropoda. 



Those that breathe air through the medium of water 



