. An introduction to zoology : for the use of high schools . olas), ortimber {Teredo). 19. The higher division of the Mollusca (tlie Cephalophora)presents a much greater range of form than do the Acephala; four Classes are recognized, each of themexhibiting important modifications of thetypical Molluscan structui-e. Most of thespecies belong to one of these, the Gas-Fig. 157. tropoda, called so on account of the de- Tellma grcenlandica. velopmenfc of the foot into a locomotive or-gan, generally a flat creejiing surface, occupying the ventral aspect of the body. Very few Gastropods retain the b
. An introduction to zoology : for the use of high schools . olas), ortimber {Teredo). 19. The higher division of the Mollusca (tlie Cephalophora)presents a much greater range of form than do the Acephala; four Classes are recognized, each of themexhibiting important modifications of thetypical Molluscan structui-e. Most of thespecies belong to one of these, the Gas-Fig. 157. tropoda, called so on account of the de- Tellma grcenlandica. velopmenfc of the foot into a locomotive or-gan, generally a flat creejiing surface, occupying the ventral aspect of the body. Very few Gastropods retain the bilateral symmetry which wesee in the Acephala ; the most primitive forms—the Chitons—do, (a group, which of all the Mollusca, comes nearest to theVermes), but in most a distinct asymmetry is present. Thisdepends upon a separation of the vegetative from the animalorgans, and the gi-ouping of the former into a visceral mass orhump, above the head and foot, which are closely united (Fig. 158).The visceral mass is protected by a shell (similar in structure. HIGH SCHOOL ZOOLOGY. 233 to that of tlie Lamellibrancbs), which, in the simplest cases, isformed of a single conical piece ; in most Gastropods, however,the shell is spiral, and the visceral mass has, in the course ofdevelopment, become twisted in the same direction, so as to causethe intestine e. g. to open anterioi-ly. This twist results in thesuppression of the gill and nephridium of one side, and also in
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1889
