The Cambridge natural history . ), operculum ; a7(, shell;V, V, velum. (After Patten, highlymaguitied.) Fig. 46.—Develoiicd larva of Ciiclas cornea L. :hr, rudimenlaiv branchiae ; hn, byssus ; /,foot ; , mantle edge ; .s7(, shell. (AfterZiegler, highly luaguilied.) of the velum itself, are the development of the visceral andshell-gland on the upper, and of the foot on the under to Lankester the primitive shell-gland does not, as a. Fig. 47. — A, Advancedveliger of Dreissensia :f, foot ; m, mouth ; ,«/;,.shell; n r, velum. (AfterKorschelt and Heider,much enlarged.) B,Acli
The Cambridge natural history . ), operculum ; a7(, shell;V, V, velum. (After Patten, highlymaguitied.) Fig. 46.—Develoiicd larva of Ciiclas cornea L. :hr, rudimenlaiv branchiae ; hn, byssus ; /,foot ; , mantle edge ; .s7(, shell. (AfterZiegler, highly luaguilied.) of the velum itself, are the development of the visceral andshell-gland on the upper, and of the foot on the under to Lankester the primitive shell-gland does not, as a. Fig. 47. — A, Advancedveliger of Dreissensia :f, foot ; m, mouth ; ,«/;,.shell; n r, velum. (AfterKorschelt and Heider,much enlarged.) B,Acligcr of a Pteropod(Tiahmannia) : y/?,oper-culum ; sh, shell ; v,velum. (After Krohn,much enlarged). rule, directly give rise to the shell of the adult mollusc, butbecomes filled up by a horny substance, and eventually disappears :the permanent shell then forms over the surface of the visceralhump from the original centre of the shell-gland. It is only in TYPES OF SEXUAL DIFFERENCE 133 Chiton, and possil>ly in Li max, that the primitive shell-sac isretained and developed into the final shell-forming area, whichis much wider, and extends to the edges of the mantle. Withinthe velar area first appear tiie rudiments of the tentacles and eyes ;when these become developed the velum atrophies and of these veligers when captured in the open sea havebeen mistaken for perfect forms, and have been described as the larva of D
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
