Archive image from page 128 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana01todd Year: 1836 in the higher forms the sexes are distinct. (See GASTEROPODA.) 17. Pteropoda, body organized for swim- ming, mantle closed above, branchiae external, no muscular foot for creeping, shell, when present, always thin, pellucid, unilocular, and inoperculate. These soft, minute, floating ani- mals are all marine, and are enabled to swim by means of two lateral musculo-cutaneous fin- like expansions, on the surface of which the respiratory branchiae or vascular ple
Archive image from page 128 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana01todd Year: 1836 in the higher forms the sexes are distinct. (See GASTEROPODA.) 17. Pteropoda, body organized for swim- ming, mantle closed above, branchiae external, no muscular foot for creeping, shell, when present, always thin, pellucid, unilocular, and inoperculate. These soft, minute, floating ani- mals are all marine, and are enabled to swim by means of two lateral musculo-cutaneous fin- like expansions, on the surface of which the respiratory branchiae or vascular plexuses are placed. These lateral fins are never supported by rays. The head is generally provided with retractile or sheathed tentacula, seldom with eyes. The body is sometimes entirely naked, often protected by a delicate thin transparent shell,which encloses tlie abdomen and is covered with a fold of the skin. They appear to be most closely allied to the inferior testaceous cepha- lopods in the nature and form of their shells and in their locomotive powers, and also in the general simplicity of their internal struc- ture, especially of their generative organs. The structure of one of the naked pteropods, clio boreulis, is represented in fig. 43, where the abdominal cavity is exposed by the mantle Fig. 43. Vfc being opened from behind. The mouth (a) leads to a long esophagus (b), which is sur- rounded by a circular series of nervous gan- glia (t). The stomach (c c) is imbedded in the lobes of the liver (g), which open by numerous short ducts into its cavity. The oesophagus is accompanied by the two long simple salivary follicles (/c), and at the left or pyloric extremity (d) of the stomach is placed the heart (j), en- closed in its pericardium, which receives the arterialized blood from the branchial veins, and sends it through the system. The bottom of the abdomen or cavity of the mantle (A) is occupied as in the cephalopods with the gene- rative organs, which consist of
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