. Elementary text-book of zoology. Zoology. MOLLUSCA. the sea. The embryos acquire velar lobes and shell, and leave the egg as larvae (fig. 528). While the velum is atrophying, the two fins gradually appear on the first-formed unpaired part of the foot, while the shell (with operculum) is usually cast off. The Hyakidce how- ever appear to keep the larval shell and develop it further, while the Cymbuliidce replace it by a new shell. The naked Pneumodermidce and Clionidce do not after the loss of the velum and shell grow direct into the sexual animal, but first acquire three rings of cilia and p
. Elementary text-book of zoology. Zoology. MOLLUSCA. the sea. The embryos acquire velar lobes and shell, and leave the egg as larvae (fig. 528). While the velum is atrophying, the two fins gradually appear on the first-formed unpaired part of the foot, while the shell (with operculum) is usually cast off. The Hyakidce how- ever appear to keep the larval shell and develop it further, while the Cymbuliidce replace it by a new shell. The naked Pneumodermidce and Clionidce do not after the loss of the velum and shell grow direct into the sexual animal, but first acquire three rings of cilia and pass into a new larval phase (fig. 529). The Pteropods always live on the high sea, but may by retracting their velum sink. /I Order 1.—THECOSOMATA. Pteropoda with a shell. Head but little developed, often not distinct; tentacles rudimentary. The rudi- mentary foot remains in connection with the fins. Fam. Hyaleidse. Shell calcareous or horny, swollen ventrally or pyramidal, symmetrical, with pointed processes. Hijalca trich-ntata Lam., Cle-odora Per. Les., Cresi'is Rang., Cr. acicula Eang., Mediterranean. Fam. Cymbuliidae. With cartilagino-gelatinous shell, boat-shaped or slipper-shaped. Cymbulia Peronii Guv., Tiedmannia luapolltana Van Ben. Order 2.—GYMNOSOMATA. Naked Pteropods, head bearing tentacles, — L&rv&ofPneumodermon often with external gills. Fins separated. (after Gegeubaur). from the foot. Larva? with rings of cilia. Fam. Clionidae. Body spindle-shaped, without gills. Clio lorealls Pall., constitutes with Limacina arctica the chief food of Whales. Fam. Pneumodermonidae. Body spindle-shaped, with external gills, and two protrusible arms, which are beset with suckers and placed in front of the fins. Pneumodermon riolaccum d'Orb. Class IV.—CEPHALOPODA. * With well-marked head, a, circle of arms bearing suckers round the mouth and funnel-shaped perforated foot. Dioecious. In the form of their body the Cephalopods are most nearly allied * Ferussac
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
