AIR-BREATHING SNAILS. 641 



transverse row, sometimes the whole radula consists of a row of single 

 teeth, one behind the other. In one group, the Elysioidea, the worn- 

 out teeth as they drop off are saved up in a little pouch in front of the 

 mouth. 



The Nudibranchiata are divided into Elysioidea, Tritonioidea, iEoli- 

 dioidea and Doridoidea. 



Though they occur on every sea-coast they are not forms often met with 

 by the ordinary student, and hence will not be discussed in further detail. 



Order II. — Pdlmonata. 



In this order are included the vast majority of the land Mollusca (both 

 snails and slugs) and of the fresh-water snails, besides three genera, repre- 

 senting a like number of families, of marine Gastropods. In all, the gills 

 have entirely disappeared, and the pallial cavity has been converted into a 

 breathing organ. The free edge of the mantle has been united to the skin 

 of the neck and back, leaving only a small contractile aperture, whilst on 

 the roof of the chamber thus formed respiration is effected by means of a 

 branching net-work of blood-vessels that surround and lead to the heart, 

 which lies with its auricle to the front. In the marine forms sea water 

 is admitted into this cavity, in the others air is drawn in and then expelled. 

 With one exception no operculum is present in the adult. 



The Pulmonata are divided into— Basommatophoka, or those in which 

 the eyes are placed at the base of the tentacles, and Stylommatophora, in 

 which the eyes are borne on the ends of tentacles. 



Sub-order 1.— Basommatophora. These have one pair of tentacles, which 

 are not retractile, and the eyes are placed at their bases. All have an 

 external shell, and, in the egg, an operculum, which is subsequently lost, 

 except in J^mp/iiboZd. . 



The AuRlcuLiDiE greatly resemble the Actfennida;— from which in all 

 probability they have descended— in the shape of their shells as well as in 

 certain points in their anatomy. The shell is oval in outline, with a 

 prominent spire and large body-whorl, whilst the aperture is long, narrow 

 behind, and widening out in front. There are two or three strong folds on 

 the columella ; the margin of the outer lip is thickened and often more or 

 less toothed on its inner side. They especially frequent salt marshes and 

 damp spots within reach of the waves. The smallest British land-shell, 

 Garychium minimum, is, however, found inland everywhere in damp spots 

 at the roots of grass. . 



The OtinievE are closely related ; their shells are auriform. They occur 

 between tide-marks, and one species is British. 



The Amphibolid.'e are represented Uy a single genus, Amphibola, which 

 has a globular, spiral shell, with an uneven surface, looking as if it had 

 shrivelled. The operculum is horny and sub-spiral. They live by the 

 margins of the sea in the Indian Ocean, Australia, New Zealand, and the 

 Fficific Islciiids 



The SiPHONARiiD^ have limpet-like shells, only distinguishable from 

 those of the true PatellidiB by the slight, channel-like depression on the 

 interior that marks the position of the opening into the respiratory chamber 

 Like the limpets they are found clinging to rocks at low water, but do not 



occur in the colder seas. . c- ^i. » ■ 'rv,„ 



The Chilinid^ are fresh- water forms that occur in South America, ihe 



43 



