Invertebrate Gallery of the Indian Museum. 129 



majority of which are terrestrial herbivorous animals, 

 though some of the Snails, eg., Limnxus, Planorbis^ etc. 

 live in fresh water, and a few Snails, and Slugs also, live 

 on rocks between tide-marks in the warmer seas. All 

 however, whatever their habitat, breathe not by means 

 of a gill, but by means of a " lung-sack" which is formed 

 by the mantle-chamber, the edge of the mantle being 

 fused with the dorsal wall of the body, except at one point 

 in front for the admission of air. Even in those aquatic 

 forms tliat breathe air dissolved in water the breathing- 

 organ is the " lung-sack", into which water, instead of 

 air, is admitted. 



In the majority of the Pulmonates a well developed 

 shell is present, but is never closed by an operculum ; in 

 the Slugs, however, \\\t shell is a rudimentary plate which 

 is generally hidden (" internal shell ") and in the marine 

 slugs [Onchidiiini] there is no shell at all. 



Ihe Pulmonata must be carefully distinguished from 

 the group of terrestrial Azygobranchs — the Pneumo- 

 chlamyda — which also breathe air by m.eans of a lung-sack 

 formed by the closure of the mantle-chamber. In the 

 Pulmonata^ or Snails and Slugs, the odontophore is armed 

 with numerous small and equal-sized denticles, the sexes 

 are united in the same individual, and the shell when pre- 

 sent, is never closed by an operculum. In the Pneumo- 

 chlamyda, on the other hand, the teeth on the odontophore 

 are in a few series of unequal size, the sexes are 

 distinct, and an operculum is developed on the back of 

 the foot. The Pulmonata are divided into two sub- 

 Orders, namely (i) the Basonimatophora, including the 

 two families of the freshwater Snails [LtmnxidcB) and the 

 marine Auriculidx, in which the eyes when present are 

 placed at the base of the cephalic tentacles ; and (2) the 

 Stylomviatophora, including the two great terrestrial fami- 

 lies of the Snails and the Slugs [Helicidx and Limacidx) 

 and the small family of estuarineand marine Slugs {Oncid- 

 iid(s), in which the eyes are borne at the summit of the 

 cephalic tentacles. The Pnhnouata are well represented 



I 



