The Cambridge natural history . \-^P Fig. 286.—Illustrating the transition Fig. 287.—Illustrating the gradual covering of form in the shell of Tecti- of the shell in the Tectibranchiata by the hranchiata from the pointed spiral epipodia and mantle : A, Ilaminea; B, to the almost flattened plate: A, Scaphander; C, Aplustrum ; D, Aplysia; Adaeon; &, Ajjlustrum; C, Cyli- E, Philine ; , cephalic disc; ejo, ep, chna; D, Ati/s; E, PhiUne; T, epipodia; sA, shell. (Not drawn to scale.)Dolahella ; G, A ply si a ; H, Pleu-robranchus. (Not drawn to scale.) additional protection afforded to the viscera

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The Cambridge natural history . \-^P Fig. 286.—Illustrating the transition Fig. 287.—Illustrating the gradual covering of form in the shell of Tecti- of the shell in the Tectibranchiata by the hranchiata from the pointed spiral epipodia and mantle : A, Ilaminea; B, to the almost flattened plate: A, Scaphander; C, Aplustrum ; D, Aplysia; Adaeon; &, Ajjlustrum; C, Cyli- E, Philine ; , cephalic disc; ejo, ep, chna; D, Ati/s; E, PhiUne; T, epipodia; sA, shell. (Not drawn to scale.)Dolahella ; G, A ply si a ; H, Pleu-robranchus. (Not drawn to scale.) additional protection afforded to the visceral mass by thereflected epipodia, which renders the existence of a shell lessand less necessary. A precisely similar line of change is seenin the Pulmonata, culminating in forms like Ario7i (p. 174). The habits of life of the Opisthobranchiata are very , especially the heavier types, burrow in sand, and are thenusually furnished with a broad cephalic disc, as a diggingapparatus; some (certain Bulla) flit about

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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895