CIRCULATORY AND RESPIRATORY SYSTEMS. 55 



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 cir- 

 culatory 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 sys- 

 tem of Acalephse; the perigastric space and pediculi of Echi- 

 nodermata; the 'atrial' system of Brachiopoda and Tunicata; 

 the aquiferous system in certain Lamellibranchiata and Gas- 

 teropoda ; 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 alternately contract and 

 dilate. The site of each vesicle is found upon contraction 

 to be associated with a number of canals arranged in a 

 radiate manner to a central point.* 



* Both vesicles lie close to the wall of the body on the side nearly oppo- 

 site to that on which the mouth opens. When expanded each vesicle is 



