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.
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.
Fig. 5.—A, A Tectibranchiate Opisthobranch, Umbrella mediterranea Lam., Naples: a, anus; br, branchia; f, foot; m, mouth; rh, rhinophores; sh, shell.
B, A Pteropod, Hyalaea tridentata Forsk., Naples: sh, shell; l, l, swimming lobes of foot.