. The animal kingdom, arranged after its organization, forming a natural history of animals, and an introduction to comparative anatomy. Zoology. 672 MOLLUSCA. d. by these animals, is the contraction of the mantle when the surface is touched, or when some irritating particle is drawn into the branchial orifice ; by this contraction a jet of water is spurted out, sometimes to a considerable distance. No beings possessed of a complex internal structure, a distinct stomach and alimentary tube, a pulsating heart, and ramifying vascular apparatus, with branchial appendages for aerating the blood, a
. The animal kingdom, arranged after its organization, forming a natural history of animals, and an introduction to comparative anatomy. Zoology. 672 MOLLUSCA. d. by these animals, is the contraction of the mantle when the surface is touched, or when some irritating particle is drawn into the branchial orifice ; by this contraction a jet of water is spurted out, sometimes to a considerable distance. No beings possessed of a complex internal structure, a distinct stomach and alimentary tube, a pulsating heart, and ramifying vascular apparatus, with branchial appendages for aerating the blood, and highly-developed secretory and reproductive organs, can be imagined to spend the period of their ex- istence in a manner more completely vegetative than these All the Tunicata above described appear to participate in a very remarkable peculi- arity in the function of circulation. The heart is very simple in its structure, being merely a contractile dilatation of the principal trunk, without any distinct division into an auricle and ventricle, or a receiving and impelling cavity. This trunk first supplies blood, as in other Mollusca, to the mass of viscera, from which the fluid proceeds on- wards to the respiratory surface, to be distributed over this for aeration ; another por- tion, however, in some Ascidians, is sent direct to that surface. The whole fluid, after being thus exposed to the oxygenating influence of the streams of water continually passing over the branchial membrane, is collected by vessels which unite into a single trunk that conveys it back to the heart. This may be said to be the direct course of the or asc'idi'a • o, branchial or'i- circulation, because it is that which takes place in the Mollusca generally. But it is ficeor mouth;*,vent; c, gan- not constant in any of the Tunicata. After the heart has constituted its pulsations glmnjrf, mantle (the external * coat being removed). for a time, so as to propel the blood which it has received from the gil
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectzoology, bookyear1854
