Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 338 COMPARATIVE ANATOMY. distinctly allied to the arrangement in the Zeugobranchia, is adapted to the asymmetry of the branchial cavity, which, again, is dependent on the characters of the shell. The smaller gill is generally approxi- mated to the other, and becomes asymmetrical in position; in some Prosobranchiata it disappears altogether (Janthina, Neritaceas, Heteropoda). The right gill is generally developed on one side only, so that it is semi-pinnate, owing to the disappearance of th
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 338 COMPARATIVE ANATOMY. distinctly allied to the arrangement in the Zeugobranchia, is adapted to the asymmetry of the branchial cavity, which, again, is dependent on the characters of the shell. The smaller gill is generally approxi- mated to the other, and becomes asymmetrical in position; in some Prosobranchiata it disappears altogether (Janthina, Neritaceas, Heteropoda). The right gill is generally developed on one side only, so that it is semi-pinnate, owing to the disappearance of the second row of lamellae. Although, as a general rule, we find the lamellar structure to be the most common, a few (Calyptra3a, Crepidula) have fila- mentous gills, and so call to mind the primitive form of the Lamelli- branch gill. The gills become modified, and may disappear altogether when the mantle and the branchial cavity are atrophied. This happens in various divisions ; thus, in the Heteropoda, among the Proso- branchiata, the gill of Carinaria is not covered over by the mantle; in Pterotrachea, where there is no mantle at all, the gill is quite free, while in Firoloi'des, the gill, as well as the mantle, disappears. Among the Opisthobranchiata the characters of the gills are equally dependent on the condition of the mantle. There is a gill on either side, between the mantle and the foot (Pleurophyllidia), or there is only a single gill in the gill-chamber, or, finally, the gill is only partly covered over by the mantle (Tectibranchiata). When the shell and mantle disappear, gill-like struc- tures may be developed on the dorsal surface of the body, as in some of the Nudibranchiata. Lamellar, or tufted and branched appen- dages, are sometimes developed in the anal region (Doris), sometimes in rows over the whole body (Tritonia, Scyllasa). If we are right in regarding the possession of a shell by the larvae of all the Opisthobranchiata as a fact, which proves conclusively
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