Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: and traversing the body for this purpose, either through a water-vascular system (q. v.), or through a system of tracheal tubes, as in Myriapoda, Insecta, and certain Arachnida. Or the blood maybe brought to the medium, either through agency of (1) vascular processes (branchiae), or (2) lungs.—(1) When the former, they may be suspended in the water, as in Pisces, the aquatic Mollusca, most Annelida, Crustacea, all larval and a few adult forms of Batrachia; or be inverted, forming a branching tube, as in Holothuria (sea cucumber). (2) When the latter, special air sacs are developed, which may be simple pneumonic chambers, as in terrestrial Gasteropoda, and pul- monate Arachnida, or lobulated, generally symmetrical sacs, opening by tubes (bronchi) into a single passage (trachea), as in all air-breathing Vertebrata. Many aquatic invertebrates have, in addition to a true circulatory system, or, with some, apparently taking its place, means of introducing and retaining within their tissues, quantities of water. The fluid, which may be considered as analogous to the serum of the blood, passes in and out of the body, through appropriate openings which lead within to more or less defined expressions of communicating vessels, or lacunae. While within the body it may become admixed with corpuscles, and is doubtless subservient to nutritive and excretory processes. Such is the fluid of the aquiferous system of Acalephse; the perigastric space and pediculi of Echi- nodermata; the 'atrial' system of Brachiopoda and Tunicata; the aquiferous system in certain Lamellibranchiata and Gasteropoda ; the water-vascular system (q. v.), etc. Protozoa. Infusoria.—The ' contractile vesicle' appears to represent a rudimentary circulatory system. In Paramecium two vesicles are present, which alternate...