. Elements of comparative anatomy. Anatomy, Comparative. INTERNAL SKELETON OF MOLLUSCA. 341 In this group the mechanism of respiration is combined with the locomotion of the animal. Each time that the muscles of the edge of the mantle relax, water passes into the branchial cavity by its orifice, that is, at each side of the funnel; after it Las bathed the gills it is driven out again by the contractions of the mantle. At this moment the cleft of the branchial cavity is closed, so that the water cannot get out except by the funnel, and this serves not only as the passage by which the water reac
. Elements of comparative anatomy. Anatomy, Comparative. INTERNAL SKELETON OF MOLLUSCA. 341 In this group the mechanism of respiration is combined with the locomotion of the animal. Each time that the muscles of the edge of the mantle relax, water passes into the branchial cavity by its orifice, that is, at each side of the funnel; after it Las bathed the gills it is driven out again by the contractions of the mantle. At this moment the cleft of the branchial cavity is closed, so that the water cannot get out except by the funnel, and this serves not only as the passage by which the water reaches the exterior, but also takes an active share in driving it out. Internal Skeleton. § 264. In most Mollusca the absence of an internal skeleton is compen- sated by the shells and tests described in § 258; for these serve as supports for the internal parts. Independent in- ternal organs of sup- port are, however, found in the Gastro- poda. Two, or some- times four, small plates of cartilage are found in the head of these animals ; they are surrounded by the muscles of the pha- rynx, and arc more or less closely connected with one another. They form the sup- porting apparatus of the radula and the parts connected with it, and also afford points to which some of the pharyngeal muscles, and especi- ally those of the ra- dula, are inserted. Cartilaginous or-. Fig. 179. Section through the head of Sepia offici- nalis. KK' Cephalic cartilages. GCerebrum, go Gan- glion of the optic nerve, w White body. I Lens, ci Ciliary body, e Cornea, p Eyelid. P Buccal mass, m External, n Internal labial membrane. e/Jaws. r Radula. oe (Eso- phagus, t Arms. gans of support are much more highly de- veloped in the Cepha- lopoda. The most important one lies in the head, where it serves as an investment of the nerve-centres, a support for the optic. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance o
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectanatomycomparative
