File:Outlines of comparative physiology touching the structure and development of the races of animals, living and extinct - for the use of schools and colleges (1870) (14759754716).jpg

Identifier: outlinesofcompar00agas (find matches)
Title: Outlines of comparative physiology touching the structure and development of the races of animals, living and extinct : for the use of schools and colleges
Year: 1870 (1870s)
Authors: Agassiz, Louis, 1807-1873 Gould, Augustus A. (Augustus Addison), 1805-1866 Wright, Thomas, 1809-1884
Subjects: Physiology, Comparative Zoology
Publisher: London : Bell & Daldy
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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rphase of development than in the hydraform group, and mani-fest an approach to the type of the tunicated mollusca. Themouth is surrounded by a circle of ciliated tentacula, the vibra-tions of which cause currents of water to flow towards theoral aperture ; the possession of ciliated tentacula forming oneof the distinctive features of this group. The mouth, situatedin the centre of the tentacular circle, leads into a long saccu-lated stomach, the walls of which are studded with glandularspecks, or biliary follicles. From about the middle of thestomach the intestine proceeds, and ascending close to its walls,opens by a rectum near the mouth (c), in such a position thatthe excrementitious matter ejected therefrom is at once carriedaway by the currents sweeping round this region ; the in-testinal canal is attached to the sac by muscular bands, andfloats freely in the visceral cavity. The tegumentary sheatiiis an organic portion of the polype, and, after enclosing 166 ORGANS OF DIGESTION.
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the internal organs, is reflected over the aperture of the cell,and becomes continuous with the tentacular circle. In con-sequence ofthis union be-tween the po-lype and itscell, it follows,that when theanimal retirestherein, thatportion of thetunic (c)pushed out-wards by theexit of the po-lype, is drawninwards on itsretreat by aprocess of in- Fig. l/o.—Plumatella repens.—a, natural size; b, the vagination, SOsame magnified. that tne flex_ ible extremity of the cell is at the same time a sheath forthe body, a support to the tentacula, and a door for closingit. In fig. 175, muscular bands are seen passing from theinner membrane of the cell to the body of the polype, bywhich the retraction of the animal and the invagination ofthe superior part of the cell is effected. At a, we see thenatural size of the polypedom of Plumatella; at b and c, thecells and polyps magnified and protruded in search of prey ;at d, the polype withdrawn into its cell, and the orifice closedby the retraction (

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