. A text-book of invertebrate morphology. Invertebrates. TYPE M0LLTT80A. 311. Fig. 139.—Nervous System of Fiona atlantica (after Bebgh from Gegeneaur). A — cerebro-pleuro-visceral gan- B = pedal ganglion. [glion. C = buccal ganglion. D = gastro-oesophageal ganglion. e = pedal commissure. e* = visceral commissure. to three such gauglia may be distinguished. In the figure of the nervous system of Fiona (Fig. 139) the concentration of the ganglia is well marked, but a decided asymmetry is made evi- dent in some forms by the exist- ence of a single parietal ganglion and of three visceral ganglia.
. A text-book of invertebrate morphology. Invertebrates. TYPE M0LLTT80A. 311. Fig. 139.—Nervous System of Fiona atlantica (after Bebgh from Gegeneaur). A — cerebro-pleuro-visceral gan- B = pedal ganglion. [glion. C = buccal ganglion. D = gastro-oesophageal ganglion. e = pedal commissure. e* = visceral commissure. to three such gauglia may be distinguished. In the figure of the nervous system of Fiona (Fig. 139) the concentration of the ganglia is well marked, but a decided asymmetry is made evi- dent in some forms by the exist- ence of a single parietal ganglion and of three visceral ganglia. In Fiona, however, the ganglionic concentration has been carried still farther, and at the same time by the suppression of the parietal ganglion as a distinct mass of cells an apparent symmetry has resulted. With regard to many other features of their anatomy con- siderable variations are to be found. Thus in some forms a well-developed spirally-coiled visceral hump is developed, while in others it loses its spiral arrangement, and in others again is elongated in the direction of the foot and can hardly be said to exist. So, too, with the occurrence of the shell, mantle, and branchiae; all are well developed in some forms, but entirely absent in others. These peculiarities will be more conveniently referred to in connection with the various groups, and it is only necessary here to refer to another feature in addition to those already given, which is common to all the members of the order—, the hermaphroditic character of the reproductive gland. This forms part of the visceral mass and is usually com- posed of numerous lobes, these again being divided into secondary lobes or acini, the lining epitheliutn of which give rise to both ova and spermatozoa. In some forms, such as BvUa and Aplysia, both elements are formed in all the acini; but in others, such as Doris, Janus, Pteropoda, etc., the epithelium of the terminal acini gives rise to ova only ; the epithelium of the
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Keywords: ., bookcentury1800, bookdecade1890, bookpublishernewyo, bookyear1894
