[Illustration: FIG. 2.--An example of the Polyplacophora: _Chiton spinosus_ Brug.] [Illustration: FIG. 3.--An example of the Aplacophora, _Neomenia carinata_ Tullb.: _a_, anus; _gr_, ventral groove; _m_, mouth.] (1) The _Amphineura_[9] are bilaterally symmetrical Mollusca, _i.e._ with organs either single and central, or paired and disposed on either side of the longer axis of the animal. The shell, when present, is never spiral, but consists of eight overlapping plates, kept together by an elliptical girdle. The Amphineura are divided into (_a_) _Polyplacophora_,[10] or Chitons, and (_b_) _Aplacophora_ (_Chaetoderma_ and _Neomenia_). (2) The _Prosobranchiata_[11] are so named from the fact that the breathing organ (branchia or ctenidium[12]) is as a rule situated _in front of_ the heart, the auricle at the same time being in front of the ventricle. They are asymmetrical, almost always furnished with a shell, which is at some time spiral, and with an operculum. The sexes are separate. They are either marine animals, or can be shown to be more or less directly derived from genera which are marine. They are divided into (_a_) _Diotocardia_[13] (_Haliotis_, _Fissurella_, _Trochus_, _Nerita_, _Patella_), which have, or whose immediate ancestors are believed to have had, two auricles to the heart, two sets of breathing organs, two kidneys, but no proboscis, penis, or siphon, and (_b_) _Monotocardia_,[14] in which the heart has only one auricle, the true breathing organ is single, and there is a single kidney. To this division belong the great majority of marine univalve Mollusca, _e.g._ _Cypraea_, _Buccinum_, _Murex_, _Littorina_, _Ianthina_, all the land and fresh-water operculates (_Cyclostoma_, _Melania_, _Paludina_, etc.), as well as the _Heteropoda_, which are a group of Prosobranchiata which have betaken themselves to a pelagic life. [Illustration: FIG. 4.--Example of a Heteropod, _Carinaria mediterranea_ Lam., Naples: _a_, anus; _br_, branchia; _f_, foot; _i_, intestine; _m_, mouth; _p_, penis; _s_, sucker; _sh_, shell; t, tentacles. × ½. The animal swims foot uppermost.] (3) In the _Opisthobranchiata_[5] the breathing organs (when present) are _behind_ the heart, and the auricle of the heart is consequently behind the ventricle. They are asymmetrical marine animals; usually, but by no means always, without a shell, scarcely ever with an operculum in the adult state. The sexes are united in the same individual. The _Opisthobranchiata_ fall into two divisions: (_a_) _Tectibranchiata_, in which the breathing organ is more or less covered by the mantle, and a shell is usually present, which is sometimes rudimentary, _e.g._ _Bulla_, _Aplysia_, _Umbrella_, and the whole group of _Pteropoda_; (_b_) _Nudibranchiata_, or sea slugs, which have no shell and no true ctenidia, but breathe either by the skin, or by ‘cerata’ or papilliform organs prominently developed on the back: _e.g._ _Doris_, _Aeolis_, _Dendronotus_.